Instructions
Play through the game's storyline until you reach Route 115. This occurs in the game after you have completed the story events in Rustboro City.
Walk around the areas in Route 115 to trigger a wild Pokemon encounter. If the Pokemon is not a Jigglypuff you can choose to either defeat it in battle, capture it or run away until you encounter a Jigglypuff. The encounter rate for Jigglypuff is 10 percent, so you may have several battles before one appears.
Weaken the Jigglypuff using your most appropriate Pokemon. Jigglypuff is weak against Fighting Type Pokemon, Ghost Type Pokemon and Rock Type Pokemon. Attack Jigglypuff until its health points have been greatly reduced.
Select a PokeBall from your item menu to try and capture the Jigglypuff. If the capture does not work it means the Jigglypuff is not yet weak enough. Continue to attack it until you can capture it in a PokeBall. Once successfully captured, Jigglypuff will join your team.