Summarize this article:

Difficult pans may require a second coating but not as much catsup. The longer the catsup is left on the bottom the greener it may become, just scrub it off and apply a fresh coat and scrub lightly. This should only take a few minutes.

Summary:
Squirt regular catsup on the bottom of inverted copper bottom pans. Let set for a few minutes. With either a good sponge or some natural charcoal, scrub the bottom of the pan until it glistens. Repeat if needed.