What you need:
-fabric flowers - I made mine using the tutorial here
-glue gun with glue sticks
-spare fabric with some weight, or some felt - I used extra crepe back satin for both projects
-pin backs for the boutonnieres
I also made leaves to go behind the flowers in the boutonnieres. I think they add a nice touch that makes it more than just a flower. These are just rectangular pieces of fabric gathered and glued on one end for dimension, and cut into a tapered leaf shape on the other.
Step 1: Making the boutonnieres
Take your fabric flower, glue the leaf to the back. Take your pin, open it up, and, using hot glue, glue it so that the pin is straight across and the leaf is in the lower left hand corner. Take a small rectangle of fabric and hot glue it over the pin back so that the pin back is sandwiched between the flower and the fabric. The pin itself should be up and out of the way so it doesn't get glued.
Once the glue is dry, you can close the pin. You can skip the extra fabric step, but I think it gives the flower more stability, and makes it less likely that the pin will pop off the back.