Instructions
Teach Hidden Move (HM) 03 to a Flying-type Pokemon. Fly to Verdanturf Town, Hoenn. Go to the Poke Mart, which is next to the Pokemon Center. Talk to the shopkeeper to see the inventory of the Poke Mart. Purchase 15 Nest balls for 15,000 PokeDollars.
Catch a "Level 5" Flying-type Pokemon like Taillow. Teach the Pokemon a Flying-type attack like "Peck." Make the Flying-type your lead Pokemon. Head to Route 116, which is located east of Rustburo City.
Explore the grassy patches of grass on Route 116. Nincada will appear 20 percent of the time in the long grass. Flee from any Pokemon that is not Nincada. On this route, the Nincada will always be "Level 6" or "Level 7."
Attack the Nincada with the Flying-type attack. Make the Nincada as weak as you can without fainting it. Navigate to your Bag and scroll down to the Nest ball with the D-pad. Press the "A" button to try to catch the Nincada with the Nest ball. Continue throwing Nest balls until you catch it successfully.