I have made literally hundreds of these. Here's what works for me. First, I never prewash my fabric. Press with Best Press starch alternative before cutting. Cut strip of fabric to size of wedge. Never cut more than four layers at once and use a new rotary blade. When sewing together, start at the wide end. Start sewing down from the top about the width of two or three stitches and then backstitch to the top, then continue to the bottom narrow end of the blade. Do NOT chain piece. The backstitch serves a couple purposes... It lets the feed dogs have fabric to start on, which avoids distortion, and it also hides the thread tails inside the plate. (Always give the dogs a bite!.) I use a stiletto to keep the bottoms lined up. As others said, sew in pairs, then add one more blade between two pairs to make a group of five wedges. Make four sets of five wedges. Sew into half circles, then combine the halves. If they are warping at the bottom, I'd bet a dollar to a donut that either your seams are slipping or you may be pushing or pulling when sewing. Let the feed dogs do all the work. Other than that, when you sew two wedges, remove from machine and check your seam on the under side to be sure nothing has shifted. Just go slow and you'll get the hang of it. If you are one that normally prewashes, don't, then when your quilt is done add a couple Color Catcher sheets to the wash. Oh and my preference is to press all the seams in the same direction from the back side, then I turn it over and press the front, using a little Best Press. This is the method that works for me. Hope it helps you. Jan