Before you begin, collect shells: they should be thoroughly washed from peels. Pour them with undiluted bleach for 7-10 hours, periodically stirring. I used Domestos. Do not bleach until crystal whiteness, shells will begin to break.

Next, take sandpaper, with fairly rough grain. Spread on a flat surface and perfect the edges of the shells to make them smooth and beautiful. Shells at this stage are still wet so it requires much effort.

Now take a round needle file and make two notches for the holes. Do not make them too big. When the shell becomes very thin, pierce it with the sharp end of the needle file and expand the hole to the desired size.

Dry option:

All buttons are ready! Then decor: you can shade them with natural dyes, pyrography, electroplating, painting different paints. These buttons can be easily washed, dyed buttons should be better steam-ironed, just in case. The dyed shells in the photo were boilng 2 hours in the solution with baking soda and walnut shells.

After decorating this way, perfect the buttons with an abrasive pad:

This is what the buttons look like on fabric: