I like to cut the fronds of leather leaf off from the main stem with a florist knife. You can tear the leaves off, but I like the sharp clean tip left by a knife cut, which I think makes inserting into the florist foam easier.
You can quickly cut enough fronds to encircle the parimeter of the bouquet holder, which will establish the eventual size of your finished bouquet.
Flower heads usually extend a little beyond the greenery, unless you plan on the greenery being part of the design as a ringed collar around the edge.
Fill in, working your way around the foam. The leaves should be inserted at a 90 degree angle from the center of the bouquet.
Fill in the center, pointing the leaf tips gradually angling forward, until the middle stems are sticking straight out at you.
This way the finishing leather greening looks dimensional and not flat, rounding out like a dome shape from the sides to the center.