As with the other two methods, the first step is to make sure the Ink layer is selected, not the Sketch layer. If you draw while the Sketch layer is selected, the black lines you draw will be merged in with the sketch, and would be hard to separate again. Keeping the ink on its own layer makes it easy to simply turn off the sketch when it is no longer needed. The cartoon-cel analogy applies here too; it's like drawing on a separate sheet of acetate over the sketch.
Now select the Paintbrush tool. In the Brush window, select a line width that is just a bit thicker than the lines you'd like to create. In the Options window, look for the "Stylus:" options. Opacity and Color should be unchecked since we only want to draw with solid, black ink, but Size should be turned on. Now, the width of your line is dependant on how hard you press.
I have inked a small part of the face with this method for means of demonstration. This
is not part of the final inking. Although the lines look a little bit less restricted
and maybe even a bit more flowing, lines that get too thin (and believe me, in a whole
piece that is inked in this method, there are a lot of them) will cause problems in
the coloring step and will need to be patched up manually.
I do recommend reading over the other two methods, and only using this one for non-vital
lines, or on pictures you do not intend to color.