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.
Place the fabric circle, oval, blob in the white button form. Put the metal button shape in next (the one without the little loop on the back).
Trim off any excess fabric from your shape; there should be enough fabric left to wrap around, but not touch/cover the center of the metal bit.
Take the metal bit with the ring on the back, and press it inside the slightly larger metal piece inside the form. It should almost "snap" into place, then use the blue press to force the backing of the button in until you feel it snap into place. It may not be an audible snap, but it won't be able to be removed easily.