FInally got around to a line tutorial. Not that detailed, but I hope it helps.
For the structure I drew a sphere to help with the volume on the middle area.
When drawing the final line art for something, think of it in parts and try to draw one stroke for it. Really think about the stroke and the direction it needs to go in.
Earlier I mentioned using a thin brush size, so I did that in the beginning. However, to add more weight onto certain areas I used a bigger brush. Look at where the light is hitting and press very lightly when adding a stroke for that area. When you're done with the basic lines, go back over strokes/areas with shadows.
Here my strokes are very curved and smooth, but you can experiment with sharp angles or just less curviness. Though for realistic lips, you really want to emphasize the curl from the tips in the middle to the corners on the end.
I don't know if you've done the plant part for Draw a Box, but I highly suggest doing that with a pen to get better with curves and adding weight to lines.
Hope this helps!
