Cut a square of any size. Cut it in Half on the diagonal. The long side of the diagonal becomes the bottom of the bag. Trim the corners and the top of the bag. At this point, decide whether it will be pieced in to the background, or appliqued on. Do so. Now, cut a bias strip from the bag fabric, following the same diagonal- about two and one half inches wide, or so. Baste the bias piece, pin to bag in a loopy way with glass head pins, and steam press it to set the curve. Gwen Marston taught me that. Sew, pulling out the pinsa as you go. Admire your not-so-Prada-bag.