Okay, I'm going to post a few steps from a drawing I worked on today. Its fairly straightforward as I think you'll see. I've included the only sketch that I ever drew for the piece to show you where it started.

Step One: Provide sketch
Step Two: Pose and render out models
Step Three: Trace (In a bright color, its easier to see) the image
Step Four: Dump the placed jpegs or whatever I was using
Step Five: Start to assemble lines into groups, groups into layer.
Step Six: Get busy refining the lines, adding texture and color etc.
Step Seven: Polish the shit out of it.

Just a note, I haven't done step seven on this image yet. Tell the truth I've barely done step six but its well on its way. The only real difference between this and the sketch is that I moved the Devil's car up to place the text below rather than above as originally imagined.

Just another note. I call it tracing, but it only starts like that. Once I have a solid geometry down it evolves into drawing as you get further and further into the finishing details.