What is the secret of putting a flannel back on a lap quilt?
I measure and pin and smooth but the flannels still streches.
the quilt is only 45 inches. these are being made for charity
and is only the top and flannel no batting in it.

Tape the flannel back to a flat surface, be firm in stretching it. Cut it a little bigger than the top. Pin, pin and pin. Then, pin a row that goes all the way across the quilt, vertical, horizontal, or diagonal. Sew that one first, slowly, stretching side to side in front of your foot. Repeat in the opposite direction. Pinning and repinning is what works for me.

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

Be sure the flannel has been washed before you use it. Flannels often shrink and bleed, but they are so nice and warm on a quilt back. I have only used flannel with batting so have no advice on how to keep it from stretching. I don't remember having that problem, so perhaps the washing will help (if you haven't already).