I do things a bit different than some of the previous ladies.
I use a 1/2 inch seam when sewing with flannel.
I don't worry about non-flammable. The warning says "not suitable for children's sleepwear", so it doesn't really apply to blankets.
I don't use batting in the middle. I find that the 2 layers of flannel are nice and cozy without being too heavy.
I also don't use a walking foot. Sounds like I need to try it out.
Good luck!