This process demonstration starts at the very beginning: the sketch. It's common to start paintings and studies with a strong under-drawing, though you might not know how to approach it in Photoshop.
One approach is to scan a traditional drawing. If that's your intention, I recommend watching this video on preparing your scanned line-art for painting.
The other approach is to draw your lines directly in Photoshop. This three part demonstration explores how to tackle the challenge. If any of the techniques are unfamiliar for you, check out the in-depth videos linked to the right.