Easy. Assuming you're setting up a sand box. Pick up the cat, carefully placing it in the sandbox. While holding him or her in the sandbox, take a front paw and dig it lightly into the kitty litter a couple of times. I've never had a cat not get it, and most have given me a look that says "why didn't you show me this before I pissed on the rug, you idiot?"
On the other hand, if you meant you wanted the cat to piss in spoiled cowboy boots, I'm not sure...