Most flannels needs to be washed 3 to 4 times before making a block so it will not stretch. You can fuse a light weight interface on the back to help with any additiona stretching. The square knot is the most common used type of knot for tying quilts but I use a wrap around the thread shank method so it does not untie after washing. Crochet thread or pearl cotton work the best. I tied my first baby quilt in 1975 and my daughter still has it after thousands of washings. Moosegirl