Instructions
Enter the Bug Catching Contest at the National Park by going to the National Park Office on a Thursday or Saturday. Speak to the man who is behind the counter and wearing a flat red cap. He will explain the rules and ask if you want to participate. If you say "yes," you will be sent into the Park with one Pokémon and twenty competition Poké Balls.
Capture the Nincada Pokémon during the course of the contest. Nincada will only appear when the contest is held on Thursdays and Saturdays. Even then, the fights against the wild Pokémon are random, so you may need to return several times to encounter Nincada. When you do encounter Nincada, battle it with your Pokémon until it's weak enough to capture it with a Poké Ball.
Leave an empty slot for a new Pokémon and an empty Poké Ball in your inventory.
Level up your Nincada until it reaches level 20. To level it up, use Nincada in battles in order for it to gain experience or give it a Rare Candy, which causes Pokémon to level up automatically. At level 20, Nincada will evolve into Ninjask, while its empty shell will evolve into Shedinja and take up your empty slot and your empty Poké Ball.