I saw that there was another instructable on how to make one of these. But requires more materials, time, and is "for girls " according to my boyfriend. We had the mouse pads with the wrist-rests already on them; but the memory foam wore out really fast and we realized it would be more cost efficient to create our own using a different filler material, and make it last longer.
Step 1: Step 1: Finding the length measurement
I used the four layers for extra protection for losing popcorn kernels, and the added comfort/cushion. You may use more or less layers depending on the type of material you are using. I used a rayon/cotton blend because it stretches easily.