There are several ways you can do Trapunto. The method you describe is one way to do it. Many quilters use water soluble thread to outline the motif with the batting, then cut away, then proceed with your normal sandwiching and quilting doing heavy background fill around the trapuntoed shape to get it to pop up off the surface. Then the quilt is submerged or spritzed with water to dissolve the water soluble thread so there is no evidence of your outline stitching. If you look at my posting
http://www.quiltingboard.com/picture...g-t256886.html I have pictured what I did. In this case I did heavy quilting in the trapunto unit so it isn't true trapunto. There are many books and tutorials that describe this particular technique in detail. Both Karen McTavish and Diane Gaudynski describe it quite well.