You can purchase the Cover Button kit at Joanns for around 3 dollars.
They also have many different sizes to choose from.
In this tutorial I used the 3/4 " size.
(I should have used a bigger size for this flower though)
The kit will have a circle on the back of the package. You will need to cut it out. This will become the template for you fabric.
Place the template over your fabric and cut a fabric circle. (You may want to cut two fabric circles if your fabric has a little bit of sheer to it.)
Make sure you center your fabric over the mold. The fabric will need to be wrong side up.
Push the button shell firmly into the mold.
Tuck the extra fabric into the button shell.
Place the back over the tucked fabric. Press firmly with the pusher to snap it into place.