If I was doing it -
1) Wash everything before doing anything - in my experience, different fabrics have different amounts of shrinkage
2) Sew the fleece pieces together and make one side of it
3) Sew the flannel pieces together and make the other side of it
4) At this point, you could do any of several things -
a) do the sew right sides together, turn inside out, and tie
b) sew wrong sides together - make a fringy edge
c) sew wrong sides together - bind
d) sew wrong sides together - fold one edge over and make a self binding