Making a boutonniere from mums is very cost effective, since each single stem generally has 3 - 5 blooms. The flower holds up very well in a cut design like boutonnieres and corsages.
Cut the heads off from the mum stem with a sharp knife. Be sure to inspect each bloom for any imperfections. The stem should be tough with a woody feel, and the petals should be firm and tight.
The number of blooms you use in your own boutonniere will determine how large it will be. If you want a smaller sized boutonniere, use only two blooms instead of three.
I like to keep the stems of my boutonniere slender and pliable. You have more control over bending the flower heads in the direction you want if you wire.
Cut the real stem fairly short (no more than 1").