Let's see...
Take a Ditto and put it at the head of your team, then go to the patch of grass on Rt. 24 (across the water from the Nugget Bridge). Wander around until you find a Pidgey, and have your Ditto use Transform. Once transformed, hit "Attack," then look at your moveset. The only attacks your Ditto should know at the moment should be Gust, Sand Attack, and one other (which I can't remember at the moment -- it's not important anyways). Now, switch Gust and Sand Attack.
Now that the part necessary to change moves is complete, defeat Pidgey. (That's right. Don't run away; defeat it. You can either have Ditto do it or switch and use another member of your team. Both work; it doesn't matter which you use.)
After the battle, feed as many Elixirs, Max Elixirs, Ethers, and Max Ethers (Yes, all four.) as you can to your Ditto (specifically, to the first move in its moveset, which should be blank) before it says that you've maxed it out.
And that should be it. You've now got a Ditto with a quirky new move. ^_^