From CosplayWiki

How to Make Vambraces

The first step to making vambraces is decide what type of material you are going to make them out of. There are many different types of materials that you can use such as wonderflex, leather, paper mache, or even fiberglass. Deciding on the material will influence how you will attach your vambrace together. Are you going to be using a clasp? Are you going to punch holes on each side and lace it up? Velcro the sides together?

Next you will need to measure. Measure the greatest and least circumferences of your forearm, and make a conical pattern. This will usually be around your wrist and around the thickest part of your forearm. Take those measurements and draw them on a sheet of paper with the length being the length you want to make your vambrace. Add 1/4 - 1/2 inch on each side to allow some space for your clothing underneith.

Note if you are making it out of leather, make the vambrace about two inches bigger than needed, to allow for the shrinkage if you are going to boil the leather. If/when you have boiled the leather long enough, wrap a towel around your arm, and use your arm as a mold for the leather. It may help if you have another person help you. This will shape the leather to your arm.