Hey Capt. Elaine!
It's easy... I turn it over before I start and stick some rolled up tape on the back (I do the edges) and stick it to a piece of cardboard or something. I leave it on there until it's done, dry and ready to go. That way it stays flat the whole time I'm working on it. Works good for me, anyway.
Welcome Valri! I'm so glad you're joining the fun!