Buy the findings in any craft/fabric store. I like the large size for my bags.
The kit comes with all the supplies you need to make the buttons except the fabric, of course.
Tip: I also use a small amount of fabric glue, it makes it a lot easier to keep the fabric from sliding on the metal, then you don't have to keep worrying if you have the image where you want it.
So first cut a piece of fabric with enough fabric to wrap around the button. I always guess and go big and it doesn't have to be perfect it all gets hidden with the button back.
Put some glue on the center of the button.
Put your fabric in place and start folding the fabric back.
Put the back on with the tools provided, or if you lost your tools use a hammer but don't hit it to hard, sometimes depending on the thickness of the fabric you can generate enough pressure with your hands to snap it on.
Easy, Easy buttons!