I like doing it by machine. I put a piece of 80/20 batting behind the section I want Trapuntoed, stitch around it with water soluable thread, after stitching I turn it over and carefully trim away the excess batt. The top is then ready to sandwich like a regular quilt. I stitch around the Trapunto sections again with regular thread and do close quilting in the background. Once finished, I spritz the surface to dissolve the first stitching and bind the quilt.