Flannel does shrink alot. If you want to wash it you need to at least zig-zag the seams before washing it to keep them from fraying. Then maybe just baste a piece of muslin or old sheet on the back of it before you wash it. I would wash the backing and the top separate. Just because they might not shrink alike. But definitely zig-zag all the seams first as much as you can. Serging would be even better if you can.