For supples I just grabbed some light colored card stock and a couple of white pre-cut rectangles which had a little rougher texture and my son's box of crayolas.
The first step is to color the entire surface of your paper with lighter colors. You could do a solid color or a pattern or even a picture if you want to, but this will end up being the lines of your picture, so most of it will go unseen.
Next, color over the top of your picture with a thick layer of black or dark blue. You could probably do brown with a teal underneath it too - there's a lot of room to experiment!
Last, find something scratchy and scratch your line drawing into the black. The black will scrape easily away, revealing the color underneath.
For a simple line drawing of a friendly robot, I used the end of a hair clip:
For Brad's more complicated sketch of Deadpool, he used a safety pin:
Tristan and I each did a scene on either side of the rectangle peices we had. I did a seascape with a setting sun and a happy whale:
While Tristan drew a sweet spacescape of the sun and Mercury. And space ships:
I decided to glue the two rectangles together:
Did a poor man's laminate job with some clear tape and scissors:
Added a tassel and voila! A bookmark!