I have a few chickens, and I am about to get some more. I also have a lot of green grass. So I had to make a chicken tractor to take care of both issues.

Step 1: Materials.

24 1x2x8ft lumber $24
1 24x 50ft chicken wire $15
1 box of 3/4 screws $6
6 packs of washers $6
4 hinges $4
2 latch $6
total: $61 for 2
$30.50 for each tractor

Tools:
wire clipers
drill w/ drill bit and phillips driver
gloves.
saw.

Step 2: Putting Together the Sides.

ends are 2 4ft pieces and 3 2ft pieces and the sides are 2 8ft pieces and 3 2ft pieces.
for each tractor you will need to cut
6x 4ft
4x 8ft
12x 2ft
plus bits for a door.

when you line up the lumber make sure to pre-drill the holes, because the wood will split and you will have to re-cut the wood again.
it will always happen on that last piece too.

Step 3: Putting the Frame Together.

this is a fun balancing act when you are doing this alone. but again pre-drill the screw holes.

Step 4: The Door.

this is a edit, as when I built it i made the door prior to adding the wire. but to save yourself a ton of headaches just build the door prior to attaching it.

Step 5: Skin It.

remember those washer? this is when you use them. start the roll at one side of the door and roll it along (attaching it with the screw/washers). the chicken wire will give you some room to goof up.
you can also roof the tractor with the same wire, I had some welded wire around so I used that.