Here's how I did it:
1. I garbage-picked a big square of plywood from an alley in my neighbourhood (you could also buy a canvas, but this was cheaper).
2. I cleaned the plywood with a damp rag.
3. I primed the "front" of the piece with a couple coats of high-adhesive primer, applied with a smooth-finish velvet roller.
4. I decided which side would be "up" in the painting, and turned the makeshift canvas upside down, so the "bottom" was at floor level.
5. I used mat black paint (left over from a variety of other projects such as my front door makeover) to brush a thick black strip along the top (which would eventually be the bottom) of the piece.
6. I filled a plastic measuring cup with the same black paint and dribbled it along the black stripe so that globs and drips would start to crawl toward the floor. I kept doing this until I was happy with the effect.
7. I blow dried the whole thing so I could install it right away (drilled directly into the wall), but even now, some of the globs are a little wet.
No matter. It looks pretty good, in my opinion. Voila: