Go to YouTube and type in "rag quilts" in the search bar and you will see many tutorials on making rag quilts. You don't have to make a rag quilt though just because you are using flannel. It is just a popular quilt for flannel as you will see in the videos.
As someone else mentioned, you can make any pattern you like using flannel. Just find a tutorial on YouTube of a quilt you like and use the flannel instead of quilting flannel. You could also make a quilt top from pre-cuts, and than use flannel on the back to add warmth to it. There are no specific rules. Just preferences.
https://www.youtube.com/watch?v=Fia6...=Tamara%27sJoy
https://www.youtube.com/watch?v=HLZy...nel=EricaArndt
https://www.youtube.com/watch?v=Dpgy...=Fons%26Porter