Ok, once you have a sketch and are ready to line art it, simply go to the piece of paper with the calligraphy pen on it. It'll make you a new layer with different options for the pen and editing tools. Once you've done that, it should look like this:
It helps if you make the sketch layer a lower opacity, especially if you're doing your lines in the same colour as the image below.
Ok, the image above is the curve tool set to size three. The curve tool is useful for if you want to keep the line the same thickness all the way through, it's also good if you need to move your hand away during the stroke, because it puts down a point whenever you click, tap or put pressure on, which you can easily undo without loosing the whole line.
The image above this paragraph is set to pen tool and size three. This reacts to pressure, so if you go light it has a thin line, if you press down hard, it does a thick line. This is also good if you want nice clean sweeps.
If you mix both types together, you can get some really lovely results. The image below uses both of the above examples, if you look closely enough you should be able to tell which is which.
