Use a brayer (usually sold for flattening wall paper seams) and a thinned out 1/3 water, 2/3 Elmer’s school glue on a plate, roll the brayer in the lightly coated plate, apply to back of the fabric shape, place where you want it, and allow to dry 24 hours to be sure there are no damp spots. It will be a little messy to work with, but once it dries you can sew right through it with no problem, and it washes out so it won’t stay stiff.