Overlapping lines are very important. Don't draw all lines the same thickness. Vary lineweight, pressure use hatching lines to draw features that have less contrast. When drawing from imagination don't hide the hands or draw mittens. Redraw them untill you are happy with them.
Most of them start in black and white. (cept the two nudes) To color them I follow the follow steps 1: give the whole thing a monotone or duotone using overlay/hard light whatever PS function 2:manually add colors with a color layer. 3: opaque paint
I see these a one shot thing. Focus and try to get everything right in one go. If it fails you start over. There's 10x as much failed sketches. I'd fill 2 A4, 192 page sketchbook per month, on top of school work and making a character design portfolio.
