for the actual EVing... you need the power item that gives EVs in that stat, pokerus, and 4-5 EXP shares to put on the other pokemon besides the one you are EVing. After you have all that, it's just a matter of switching pokemon in to battle, repeating until enough EXP share pokemon have entered the battle to make the power item poke gain 1 EXP, along with 8 EVs in the power item stat + 2 in speed from killing a single magikarp