It might be different for other cartoonists, but for me, it's a long process. For starters, I need the idea. Luckily, they come to me pretty easily.
Some cartoonist like to isolate themselves when they come up with ideas.
Not me, I like to get out and see things. That's one of the reasons why I like working in a big city like New York. I almost never sit down at my drawing table and force myself to come up with a joke. Once I have any idea, if I don't think that I will remember it, I'll take out a pad that I always carry and write it down. If I need to remember how something looks, I'll do a quick drawing of it.