These are the same size as regular shirt buttons and were inspired by this post on Craft:blog and these much larger buttons from ABC Buttons.
In brief the steps are: 1) Make an original by lathing some jeweler's wax to shape. 2) Drill the grid of holes (using the technique from this Instructable). 3) Make a silicone mold of the original. 4) Make a cast of the button in polyurethane resin. 5) trim and sand the back of the button smooth. 6) repeat steps 4 & 5 a bunch of times. 7) sew.
Thanks to scoochmaroo for taking the time to test them in an actual sewing environment. (And making an awesome gift with them.)
There are a few more words on the process on my blog if you're so inclined.