There are a few on-line instructions for making prefolds. If you have one at home, you can easily measure it and make one from there- it is basically just a 'sandwich' of two materials with 1/3 in the middle having more layers of absorbent material in the sandwich.

think about the type of materials you want to use- do you want to use snappys? if so, you need material they will grip.
you want something absorbent in the middle

if you want them to fit into certain covers, measure the cover from back to front ad this will give you the height of the prefold.