So this is a new technique I'm playing with.
Beginning with a sketch, keep it rough knowing this isn't going to be the final thing. When satisfied with the general shapes and composition I trace the sketch using watercolour pencil and ink on Matte Dura-lar Film. Something about it gives a really beautifully crisp line. I can also erase really well because the surface is super slick.
I then add another layer applying more colour and line-work and perhaps some details that weren't present in the sketch.
The Matte Film allows me to play a little with depth, fading and detail reduction.
You can continue to add layers as you go, a little bit like photoshop, but analogue style. Pretty cool, right?