I made a short, quick, and dirty explanation about how I approach lay-ins for a different forum. I'll post it here in case someone here will find it useful (or will be able to point to problems with this approach).
Here are the pointers:
- Use light, thin lines
- A good lay-in already has the sense of 3d without any tone
- Avoid continuous contours, always look for overlaps
- Draw through the form
- Think in 3d (study the skull, the Loomis method, whatever else helps)
- Make sure the outline has the same feeling as the original (I failed at this step here)
- Center lines are your friends
- Always check and re-check alignments and basic proportions