When you see that [the tilde symbol] it means you can edit the shape by drawing. Holding the left mouse button down you draw freehand. When the cursor reaches another edge the tilde symbol will reappear and upon releasing the mouse button you'll find part of the shape has been trimmed away. I find it works best to just nip away small amounts repeatedly rather than trim all the way around in one step. Note that if you trim away too much you can use the same technique to add it back - just by editing the shape 'bigger'. Also note all the normal functionality of the Freehand and Brush Tool applies. You can for instance adjust its smoothness slider to modify what you've drawn: higher settings for a smoother line.

Proceed around the shape nipping off bits you don't want. It's easy. This Freehand and Brush Tool method works particularly well when controlled by a tablet. It makes it almost as easy as using a real pencil. (Tablets also make good presents to yourself)!