Sandy Bonsib has a book of patterns for flannel quilts:
https://www.amazon.com/Flannel-Quilt...dp/1564773604/
If you search on "flannel quilts" on Amazon, you will find a number of books for pieced flannel quilts.
What I would do is wash and dry the flannel in a dryer at least twice, then starch heavily (1:1 solution of Sta-Flo liquid laundry starch and water) and iron before cutting. Some flannels shrink a lot while others do not shrink at all. Pre-washing and drying a couple of times will even out those differences. Flannel is very limp to work with. Starching before cutting helps you get accurate cuts and also accurate piecing.
Just in general for pieced flannel quilts, it's a good idea to choose relatively large, simple shapes over a block that is very intricate. Log cabin can work well with flannel. If the pattern allows it, 1/2" seams instead of 1/4" seams help ensure that the seams don't fray out. You can certainly use 1/4" seams, but in that case be sure that there is adequate quilting to relieve stress on the seams when the quilt is used.