Adding interesting foliage makes for a more interesting corsages and boutonnieres. Although some online websites insist you buy at least 10 bunches, I allow you to buy in affordable quantities (many times single bunches) so you can mix and match like a professional florist would.
Some green leaves need additional support. Tape a supporting wire with floral tape.
Create a loop and lay it against the back of a galax leaf.
Tape down the stem with the open loop at the back. This will hold the leaf up in a forward manner, framing the boutonniere flowers.
Now the leaf stands up with the support and you are ready to back the carnation with it.
Place the greens against the back of the flower and tape as before.
Whether or not I clip off the extra stem depends on whether I want the stems to be part of the design or whether I want a single stem. Clip off extra bulk if you want a slender, single stem.
You can see that the galax leaf cups around the flower head with the support wire holding it in place.