Before we continue make sure you're color is set to white:
Now in your Layers palette, you should see 2 other Tabs: Paths and Channels.
Now I've dragged out Channels, as I like to have that seperate. So don't worry if my screenshots look a bit differant
Go to the Paths tab. You'll find that path we just drew, right click on it and pick Stroke Path.
Another window comes up, set it to brush and hit OK.
Here's my result, the Brush nicely followed our little path, no need to this kind of thing by hand
Nearly done, but we need to clean this up a bit.
Go back to Path palette and right click, this time pick Make Selection.
Paths are real handy to make good selections. I'll show more of them later on, as they can be a bit tricky to get used to. Mainly on the curve part.