Below you’ll see that I’ve completed the trace of the general shape of her eye. You can go back to those little nodes at any time and do some tweaking until you’re satisfied with the shape. Make sure your paths are closed (although in some cases it is not necessary).
What you want to do now is right click (Windows) / ctrl click (OS X) somewhere in the canvas and select Fill Path. Fill it up with black, your foreground color.
After filling your closed path with color, you can delete the path you’ve made. Below is the result of what I did so far. At this stage, you can go ahead and trace the rest of the face’s detail, like for instance her eyebrows and irises.
I use this tracing technique particularly on the eyes, as I am meticulous with how my drawings’ eyes are shaped, and for me the best way to achieve what I want is trace and fill the path.