So I lay the foam on the fabric and trace for the top plate and then flip the foam and trace the bottom plate. Am I adding seam allowance? The cushions are about 5.5 ft long and 3 ft wide at the widest point. Where am I making my hash marks, since the top and bottom plates are different sizes? Also. Is there anything special to know about the boxing, or am I just cutting the proper length? Thanks in advance. I do these so infrequently that I'm unsure of myself.