Bind off the same way you always do. However when binding off the edge of a hat like that you need a stretchy bind off. Here's some. I've done the k2tog and it was pretty stretchy. Some people say the sewn bind off is the stretchiest, but I haven't tried that one yet.
Here is a tutorial for dealing with that last stitch when binding off in the round. I haven't tried this, but I will next time!!