Bonnie Hunter has several free patterns. Bricks and Stepping Stones or Dancing Nine Patches would work well in flannel, I think.
I don't normally wash fabric before using it, but I definitely do with flannel. There are also wide variations in quality. Big box store flannels tend to pill and get scratchy. I'd look for a good sale elsewhere. The last ones I bought were from Hancocks of Paducah. Very heavy and vey soft.