We used a lovely kit for ours made by Armommy which is a basic fat quarter. You can make your own by tracing out 1 1/2-2 1/2 inches each on any fabric of your choice. The white plastic guide is included in your kit.
You will need:
plastic press and form (white and blue pieces)
Step 1: Cut Out Fabric
Each button kit should have a rough size for you to cut out. One inch, one and a half inches, three inches; something along those lines. Cut out a circle, or rough circle, of that size.