Men wear boutonnieres for formal occasions such as weddings and proms. Usually they are attached to a jacket lapel, which has more heft than a shirt. When you put one on a shirt, make sure you pin only the shirt and not the guy wearing it.
Place the boutonniere on the left side of the shirt, about 4 inches below the collar bone.
Hold the boutonniere in place with your non-dominant hand and hold the pin with your dominant hand.
Stick the pin through the shirt carefully, making sure not to stick the wearer. Send the pin back through the shirt. Put the pin through the stem of the boutonniere. On the other side, send the pin back through the shirt and back outside again.