Whether it's the:

1. scoop-in-the-back because you have lovely curves

2. after wearing for two hours they stretch out to what seems to be a whole size

3. if you've lost a little bit of weight

... jeans CAN be resized to look like they weren't. In this Instructable I will show you the correct, mistake-proof way to achieve the perfect-fitting jeans!

Step 1: Girls and Boys Alike!

Jeans are mostly resized at the back. There are rivets on the side pockets that get in the way of alterations. Generally, when resizing clothing, there are limits to how much you can take things in before distortion occurs, or it just doesn't make sense. I've done thousands of resizes in my business and I don't like to say no, so I have certainly pushed the limits, but a guide is as follows:

~ jeans/trousers - no more than 2" off of the back. Most of the time, except for jeans, you want to resize the sides, for proper symmetry and drape. You can certainly adjust the legs though, on both types as well, to narrow the hips or to skinny a boot cut.

~ dress shirts/ jackets - up to 6" total(3" off each side)

~ skirts - up to 4" total, maybe 6 depending on style. You may have to adjust the darts, etc.

(If I get enough requests in the comments, I will do an Instructable on any or all of the above. I do it all day long and it's always fun to make a detailed tutorial on it.)

So, back to the jeans... that is why you are here!

Take in more than you think you need, in case they are stretchy, but not so much that after expanding from wearing you still have a muffin top. : P Generally one to two inches makes a world of a difference!

~ The first pair is a pricey bedazzled Miss Me pair of ladies jeans. The customer has lost some weight so I will take them in 1.5" That is the limit for these... you don't want a uni-pocket!

~ The second pair are mens Levis. They fit him okay when they are straight out of the washer/dryer, but after a few hours of warmth and movement they expand much too much to his dismay. Those I will take in 1".

What you will need:Seam ripper, needle and thread for basting, iron/board, ruler, pins, thread scissors, shears(big scissors), sewing machine(s), appropriate thread for project, chalk, good lighting, patience.

Use the seam ripper for the next several steps(Respect the seam ripper; it can be your best friend and worst enemy at the same time. Use good lighting, go slowly and pay attention.)

remove back carrier(s) and pin to pants with safety pin, not stick pin(you will get stuck!) so you won't lose it.(pics 3,4)
remove all stitching from the center back waistband, 5-6", depending on size (pic 5)
undo entire vertical back seam and remove all thread.(stop at horizontal crotch seam) This stitching is usually a double row and is almost always a chainstitch, which means if you cut it just so, the whole thing unravels. This works best with new pants. If you've worn them quite a bit then they've gotten a bit funky(nothing personal!) and the thread connections are more bound up and less likely to unravel easily, so those will take longer.(pic 6)

Step 2: Iron, Baste, and Don't Get Nervous

1. Press out all seams and fold lines. They are pretty stubborn so you won't be able to completely remove them, but the better job you do now will make for less confusion later.

2. Find the exact center back of waistband and baste a thread line with some colorful thread, from raw edge of bottom to raw edge of top. Usually the stitching holes from the carrier are a good guide but they are not always centered perfectly, so double and triple check. Then baste a different color thread line for where your resize stitching will go. Do that on the inside and outside if there are two waistbands.

3/4. Now chalk out your cutting lines along the butt seam. Since there are two seat halves, you halve the resize and draw a chalk line on each half that measurement, from the top, slowly decreasing as you approach the crotch seam, where the resize should approach zero. (So my 1.5" resize was .75" away from the raw edge, most of the way down, until I approached the crotch, where I decreased to .5", then .25")

5. Ready? Cut along the line! Be brave, you can do it!

After this point, there is no going back. If you catch yourself eating those cookies, immediately follow-up with 12 squats, 20 crunches, 7 push-ups. *

* Disclaimer: this may not actually work. And if you double up on those exercises so that you can help yourself to another cookie, that may or may not work either.

Step 3: The (modified)Flat Fell Seam

Usually these are done as you are constructing a garment, and not after the fact, so we have to get a little tedious and creative here. There are a few ways to do this, and they even make special feet for this, but we don't wanna get all fancy here so I'm going to show you what I think is the easiest for anyone, and foolproof.

Using the crotch seam folds as a guide, fold each raw edge to continue that, 3/8". Press with hot iron(watch your fingers from the hot steam!) You want very pronounced new lines so as not to get confused with the old lines.
Now, starting at the crotch seam, lay the top fold over the lower fold, slightly overlapping the raw edge by a good 1/8".(technically a flat-fell seam gets sandwiched inside of each other, but you'll go bananas trying to do that. Try it if you'd like... good luck and have fun! This method will work just as well.)
Pin in the middle, catching all 4 layers(this is important). Face them in the direction I have shown; they will be easier to take out as you sew. At this beginning point you are not really changing the size, but be sure that your fold is over enough to cover the old stitching holes. This shows that you are right on track. Continue working your way up, paying attention to your new folds that you pressed which define your 3/8" seam, and not the old folds/seams.

Are you ready to start sewing? Will it be scary? A little... but if you follow my advice to prep your machine to it will be good and ready.... so should you!!

Step 4: But One Thing First....

I'll try to watch my language here... if you have a sissy machine then this may not be fun for you. So let's go back in time here, before you dismantled your jeans and do a little test if you are not sure. Follow my guide for setting up your machine and do a test run over 4 layers of jean fabric.

I have a Viking straight stitch machine that I do my heavy work on. For this test I'll step down to my basic Janome for full empathy and no b.s.(but I'll be doing the actual, post-test work on my Viking, thank you very much.)

assemble the following: Thick thread the same color as the jeans used, a new jeans needle, some scrap jean fabric, and an empty bobbin. And maybe a hammer. You'll find that pounding down some bulky seams works wonders!
Load needle and thread spool onto machine, wind a full bobbin with the thick thread, and make some test runs through the denim, four layers thick. You want to experiment with your settings. I had to adjust the spool tension to just over 8 and the stitch length to 2.8. All good with the four layers? Now introduce a small perpendicular bump to see if your presser foot can ride over that without fussing too much. Wear goggles if you want. :D If you go slowly and your machine is computerized it will stop, because it is trying to protect your needle. The trick is to go at a pretty good clip while holding the front and back of what you are feeding through taught and slightly elevated. Not too tightly, but, well, you'll just have to get to know your own machine a little bit, if you don't already. This elevation simulates a jean seam jig, which you don't have the time for, literally. Your machine will whine or stop because you are going slowly and still probably skip stitches. If you manually hand-feed it, it won't stop or break, of course, but will skip stitches. Momentum is almost always your friend.

Step 5: Now That We Settled That...

Let's begin!!

Shimmy pants under presser foot until needle is just above and a bit in front of horizontal crotch. Make sure sewing area underneath is clear of front of pants. We want to sew that folded, outside edge first. Lower needle.
As you begin to sew, remove needle that is coming up. Continue sewing, doing your best to sew 1/16th from that folded edge in as perfect a line as you can. When you head back south this will be your guide line that you can just line your presser foot against for a consistent width. The Miss Me jeans had a very long double stitch, so when I got to the top of the pants I went back down again, then went across 3/8" to start sewing the other parallel line. Make sure to peek underneath to ensure that underfolded edge is still folded in place. Begin to sew back up the pants, using your presser foot as a guide to keep the two lines as parallel as you can. My machine is a straight stitch so I can't adjust the needle position, but feel free to adjust yours so that the foot lies flush against that first line. (Alternatively, you could have stopped and flipped the pants over and sewed from the inside to better sew the edge of the underfold, but the bobbin stitch may not look the same as the needle stitch.) If you ironed and pinned nicely it should catch everything just fine.
Oh, and as you approach those horizontal yoke seams, remember the elevation trick, and also know that the presser foot is not only pushing down but also pushing the fabric forward. Take this into account and adjust the placement of those overlapping yoke seams to be sure they are sewn mostly on top of each other, and not staggered because you stretched the top fabric as you were stitching.
On your way back down, continue to peek and or feel underneath, making sure that the fold has stayed in place. As long as you sew a good 3/8" away from your first line, you'll catch and embed everything. After you are done, if you look underneath, is may not look perfect, factory finish, that is because they use a special foot and it is done closer to the beginning of the construction of the garment. As long as your outside, visible lines are even and have caught everything in the layers, be proud of your nice, strong work.
sewing note: normally this is done with an industrial chainstitch, for flexibility and ease(they have no bobbins) But since we are using very strong thread and not locking/backstitich, plus topstitiching, they really should be just fine.

Step 6: Resize the Waistband, Connect to Seat, and Carrier Attatchment

Now let's resize the waistband, connect it back to the seat and sew the carriers back on

Flip band inside out, evenly line upf folds/thread resize bastings and pin about 3/4" away from sewing area at both top and bottom. This is easier with the Levis because I only have to do it once. This is also why you opened up that seam as much as you did. Any less and it would be a pain to turn out.
Decrease stitch length and stitch, back locking at beginning and end. Clip threads Clip triangles if this is a single, fold-over band. Remove all basting and check fit with yoke. You should have to stretch the waistband a tiny bit for a perfect fit.
Press open ends where old foldlines were and fold over and press to create new ones. Be sure not to change the height size of the waistband. There should be a fade line to use as a guide.
Give the completed, folded unit a good press and bring back to sewing machine. Increase stitch length. Stitch top of waistband, sewing over original stitching at beginning and end by .5". Repeat with another row. The goal here is to make a visually smooth transition from the old thread to the new thread. (if you have a 2-piece waistband then you'll just have to do this step twice.) tip: You can roll/fold one leg to create an on-the-spot ironing ham for that narrow waistband.
Repeat for bottom, tucking yoke of seat inside. Check placement of all folds and make sure you are happy with it. You may have to stretch the waistband a bit to fit. That is perfect. There's another bump in the rode here so be brave, use the holding both ends and elevation trick and you'll be good. Repeat another line of stitching if you'd like. The Levis had a different color so I switched to dark grey.
The carrier. Okay, no joking.. this thing is thick so you may not be able to go through it with your machine unless you use the handwheel. You can't get momentum on such a tiny thing. Hammering everything will help immensely, though. Place exactly where the original one would be, centered at the top and below the waistband. Lemme check with my Janome. ....Yeah, you should do it by hand, turning the wheel manually. Small stitch length, about 5 stitches, then reverse by hand(or manual pivot to go forward again if you have a computerized) stitch again, then one more time, and hit both sides of top and bottom with fray check. See, what came out of the factory was 2-4 layers of denim, plus another 4 with the carrier, to what is now another 2 layers on top of that because you split the seam. You're asking for a lot!!
Or just glue it, with a good glue. This is probably what most people do. You should still have remnants of the old stitching on there, so it will look the same.
The final detail... sanding! Just use a medium grit. The foam sanding blocks are good to have on hand. Nothing too crazy, you just want to mask any obvious undistressed transitions.
And that's it! You're done! If you made it this far with no broken needles or re-do's then you did an awesome job. I want to see your pics of what you did! And please ask any questions if you have them!!