Instructions
Obtain a Pokemon that is currently infected with Pokerus. A Pokemon that is cured of Pokerus will not be able to spread Pokerus to other Pokemon. You can get a Pokemon with Pokerus by trading with someone else who has already come across Pokerus.
Put the Pokemon infected with Pokerus in your party. Catch five other Pokemon and place them in your party. Fight trainers or wild Pokemon in caves and long grass. After a few battles, all your Pokemon will have Pokerus.
Place the newly infected Pokemon into your PC. After a few days, a Pokemon with Pokerus will be cured of the disease. Once a Pokemon is cured, they will still receive the EV benefits but they won't be able to spread Pokerus to other Pokemon. Place a Pokemon in the PC and they will have Pokerus for as long as you keep it in there.
Breed two Pokemon at the Daycare until you get an egg. Place the egg in your party, along with a Pokemon that has Pokerus. Battle a few Pokemon and the egg will have Pokerus. An egg infected with Pokerus will hatch into a Pokemon infected with Pokerus.