All it takes is about a yard of fabric to make a cute, custom pillow case. My sewing skills are very lacking, but this is all straight lines, so it is super easy. All you need is:
Fabric (I used about a yard of one, and about a quarter yard of another.
Step 1: Make a "Pattern"
Using a pillowcase you already have, measure the rectangle to find how big you need to make yours. Mine was about 20 inches wide and 30 inches long. Afterwards I googled the "standard pillowcase size" and found that most are about 22 inches by 32 inches.
I decided I wanted a little bit of contrasting fabri on the edge of mine, so I decided 5 inches of the 30 were enough. I added 1 inch seem alowance on all seems, and an extra 3 inches to fold over on the open edges, since that seems to be standard for all pillowcases. So, I came to decide, I need 2 pieces of the "main" fabric, 22 inches by 27", and 2 pieces of the "accent" fabric that were 22 inches by 9 inches.