knitting question

So I'm working on a scarf in stockinette stitch for a Christmas present and it's curling like crazy on the sides and slightly on the bottom. And I'm at a loss for how to get it to lie flat. Is there anyone who could please give me some advice/tips on how to fix this? And should I sew in the yarn tails (since I'm using a couple of different colors) before or after I do whatever it is I need to do? Or does it not matter?


Thanks very much!