Here is a collection of
buttons taken from vintage and antique waistcoats. On the left are the
shell and mother of pearl buttons, and the black ones on the right are
made of glass. In Victorian times, many garments had their buttons
removed for cleaning and pressing. Since the buttons are not
permanently sewn on, this also means you may interchange them, and wear
your favorite set of buttons on every waistcoat, if you like.
First pick out buttons that will fit your button holes; hold the button
next to the hole and make sure the diameter is just a bit smaller than
the button hole. Then find the little eyelet holes where the buttons
should be, and push the metal shank that comes out the back of the
button, through the hole. Once it is through, a metal ring (like a
modern keyring) winds through the hole in the button shank to hold it
in place. Or, if your waiscoat is after the Industrial Revolution, it
probably came with cotter pins for this purpose.