Attaching the leaves to your boutonniere is not difficult once you learn how to wire every single leaf so you have more control over the bend and sway of the stems.
You can simply tape the leaf as it to the back of the unwired flower stem, but I like the control that wiring both gives me in being able to gently bend the flower to the way I want it to look.
Bend the head of the flower gently forward, so that when laid against the tux lapel the face of the flower faces outward rather than upward.
The leaf should be place straight behind, bending only if necessary to make the leaf cup the back of the flower head. Keep the tip of the leaf a bit longer than the actual flower head, so that it frames it nicely once pinned to the lapel.
Use more florist tape to secure the two stems together. Tape only a short way down however - not the entire length of both wire stems.
Once you've taped far enough to securely hold the leaf in place, stop and keep the stems separate. As a professional florist, I always try to keep my flower stems on corsages and boutonnieres slender and manageable. Big bulky stems have an unprofessional look and are sometimes difficult to pin.
Using wire cutters, snip one of the stems off in a clean cut and then continue taping down the rest of the stem. Sometimes I do not cut off the extra stems in order to have several stems to curl up and leave a decorative finish.
If you do not plan to include the stems as part of the design, work at keeping the finished stem thin and bendable.
You'll have a smooth, slender stem with a tightly taped finish all the way down covering all the wire.