Instructions
Travel to Route 116. Located in between Rustboro City and Verdanturf City, this is the only area in the game where you can catch Skitty.
Encounter a wild Skitty. Walk around in the tall grass until you find one. Doing so may take some time, because Skitty has a very low encounter rate.
Catch the Skitty with a Poke Ball. Weaken it until its HP bar is red, for the best capture rate. If you have any attacks that cause sleep or paralysis, use them to further increase your odds of success. Keep in mind that Skitty's "Cute Charm" ability can cause your Pokémon of the opposite gender to fall in love with it. To prevent this, avoid physical attacks, or use a Pokémon of a different gender.