I usually just uncurl the edge I'm working on and do the mattress stitch. The stitching stops the curling and no preparation is necessary, in my experience.
I supposed you could wet it to flatten it out and it wouldn't be a problem at all since you're going to be felting it anyway.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry