if you have already cut your squares you really don't need sizing or starch---that is generally used before cutting - it helps keep the flannel from stretching while cutting.
i do not (baby) my flannels- and make many many flannel quilts- i toss them into the washer on a regular cycle- dry them in a hot dryer- use a bit wider seam allowance- and if they are heavy-double sided really good flannels i lengthen the stitch length a bit- too small of a stitch may cause a little bit of puckering along the seam line if they are heavy-
other than that use like any other cotton fabric- that's what it is- just softer/loftier - and in my book much nicer to work with....some people complain about flannels being stretchy---in 48 years of sewing i've never experienced this problem- i have seen some thin-loose weave flannels that were stretchy - but i never bought them so have no experience with them.