if you stretch your flannel as you sew you will have some pretty good puckers---
you should smooth it flat-without stretching- sew with the flannel on the bottom-so the feed dogs keep it traveling along smoothly. do not pull on it as you stitch. the flannel (usually) holds the other fabrics in place fairly well without pins- a 1/2" seam is good to use with flannels- they tend to fray more than regular cottons due to the looser weave. stitch slowly- with a little bit longer stitch than normal