I always shape it first. A put a running stitch through one row of the diamonds and cinch it together and tie it off. I glue that part underneath the rose or whatever I'm using it with. Then place it over the piece and you can do the same on the other side or leave it sticking up depending on how big it is and what look you're going for.
When the entire piece is finished, back it with felt for softness and visual appeal.