I've done a couple of pewter badges already, but I came across this instructable, and I realised that I had been over-complicating things.

Here's my take on Windswept and Interesting's project...

Step 1: Needful Things

I made this with;

Scrap MDF (3mm thick is plenty)
A Stanley knife
A fret saw
Clamps
Pewter melting equipment. As I currently teach Design Technology ("shop") in a small high school, I have access to a dedicated pewter melter, but you can use a blowtorch (as Windswept did) or hot-air gun as I did in my previous projects, and some sort of metal ladle. With the "proper" pewter melter, I still use an old spoon to pour the pewter.
A brooch or badge back
Superglue.

Step 2: Scribble...

The would for this broach needs three layers of MDF, so roughly divide your scrap into three.

On the first section of MDF, sketch out the outline of the broach you want to make. No need to be precise, this is art...

Include in your sketch a V-shaped notch to pour in the pewter, and enough narrow channels to let the air out.

Step 3: Cut

With the fret saw, cut out your sketch and the V-shaped notch.

With the Stanley knife, cut grooves where you marked the channels to let out the air.

Step 4: More Sketching

Use the cut-out piece to mark the same shape onto the second section of MDF, and sketch in the area you want to be textured.

Step 5: Carving

Use the Stanley knife to cut around the edge of the area you want textured, then peel and pick out some of the MDF. Stop digging when you feel like it, but make sure it's before you go right through!

Step 6: Line Up and Clamp

Like the title of the step says, line up your cut layers of MDF, add the the third layer (the one you haven't cut), and clamp them firmly.

To make your life easier, set up the clamps so that the mould stands vertically without you needing to hold it.

Step 7: Pour...

When you are ready, and the pewter is melted, pour it into the V-shaped notch.

Watch carefully, and stop pouring if it overflows.

Step 8: De-mold

Give your pewter time to cool and set, let off the clamps and break apart the mould.

It will probably still be very hot - wear gloves, handle with pliers, or leave things longer to cool.

On the first pour, I found that the mold was only part-full.

I think that either the V-shaped notch or the air channels were too small, so I made both slightly larger, and dropped the failed cast back in the pot.

The second pour worked perfectly, and produced a cast with a lovely texture.

Sometimes the mold can be re-used once or twice, especially if the outline is a smooth shape and the texturing is light, but the heat of the pewter slightly chars the MDF, changing the texture so that every piece you produce is unique. If the mold is complex, you may need to break the MDF apart to get the broach out, but it is so easy to make a new one, that's not really a problem.