I am not a PRO at hand applique, as a matter of fact it is my worse technique, but I have done a few projects using this technique.
I, personally would not use any type of bonding agent because it will make the stems a little too stiff, especially if you are hand appliquing everything else. Roxanne's Glue is just a glue that holds down fabric, it looks like Elmer's Glue, but it is for fabric. If you cut your fabric on the bias, you shouldn't have a problem shaping it the way you want it to be. If there are curves in the shape, snip the curved edge before tucking the fabric under to eliminate bulk and to help make the shape.
Hope this helps some.