Things You'll Need
Instructions
Visit the National Park on a Thursday and enter the Bug Catching Contest to catch Nincada.
Buy a spare Pokeball of any type you like.
Deposit one of the members of your party in a box at the Pokemon Center, so that you only have five Pokemon with you. To get Shedinja you will need an extra space available in your team.
Level Nincada up until it reaches level 20. You can do this either by battling or with Rare Candy, if you have any available.
Allow Nincada to evolve when prompted. Nincada will evolve into Ninjask, and will also produce a Shedinja that will take up the open slot in your party -- and use your extra Pokeball.