How do you capture a wild Wobbuffet on Pokemon Ruby?

You can't capture a Wobbuffet in Pokemon Ruby. Wobbuffet is a Gen II Pokemon, meaning it was introduced in Pokemon Gold and Silver.

While you can trade in Pokemon from other games using the Game Link Cable, Wobbuffet itself can't be obtained in Pokemon Ruby without cheating.

If you're looking to capture a Wobbuffet, you'd need to play Pokemon Gold, Silver, or Crystal, or obtain one via trading in a later generation game.