The key is the pinning. For blocks that are too small, pin the two corners first, stretching the block to match the length of the sashing, then pin the middle and then again at the 1/4 and 3/4 marks. For blocks that are too large, do the same thing but with the stretching done on the sashing instead of the block. When you sew them together, put the longer piece on the bottom (as in underneath).