Flannel in the US is just made with cotton, so you can use whatever kind of starch or sizing that you prefer. I currently use Faultless Starch (I have a severe allergy to sizing, but it works just as well). I've also mixed my own with Sta-Flo. You want to use a decent amount of starch to help give the flannel a bit of stiffness. I usually spray a generous coating over the entire right side of my fabric, let it soak in for 3-5 minutes & then I iron on the wrong side of the fabric (keeps starch from building up on the sole plate of the iron). I repeat that process in reverse & then cut. If my fabric has been sitting for a while before I have a chance to piece it, I will just spray a light mist of starch on one side & steam press before sewing. The moisture will help "re-activate" the starch.
Note: If you live in an area with a warm, moist climate, you want to be cautious about using starch as it can attract silverfish & other pests. I know quilters down in Florida who switch to sizing if their project is going to sit out for a length of time before it gets finished & washed to remove starch. I've never had problems here in Illinois.