This is a LOT of extra work, but I just make pattern pieces of each shape, at a quarter inch all around and cut it out as if I were doing regular piecing. That also helps me when I add on the next piece, since I can use the quarter inch outside the seam to line up my next piece. Just remember to cut your fabric in the reverse, so when you place it on the paper and flip it over to sew it, everything lines up.