http://www.quiltingboard.com/posts/list/824.page
Check here for pic and links. They are easy to make. No hard and fast rules. Use a foundation block of muslin or whatever, sew on strips of fabric from one edge to the other. Place 1st and 2nd strips facing each other and sew in place. Finger press open. Place another strip, right sides facing, sew and finger press. When you have the whole block covered, trim off the excess fabric. If you miss a spot, fill it in with another patch. Easy as pie.