Step1: You must find a Pokemon with PkRS status.
If you don't already have one, you must ask someone to trade one to you.

Step2: Infect the Pokemon that you want to EV train by battling wild Pokemon using the Pokemon that is infected with PkRS.
Make sure the Pokemon you want to infect are in the party.
Battling with a Pokemon infected by PkRS will spread to the other Pokemon in the party.
PkRS doubles the EV Points gained when knocking out a Pokemon.

Step3: You must purchase the 6 Power Items
These Items are found at the Prize Exchange, near the Battle Tower/Fight Area.
You must gain the total amount of 96 Battle Points (BP), by battling in the Battle tower/ Battle Frontier.
The Power Items will cost you 16 BP each.

PowerBracer: Atk
PowerBelt: Defense
PowerLens: Sp.Atk
PowerBand:Sp.def
PowerAnklet: Speed
PowerWeight:Hp

Step4:Give your PkRS infected Pokemon one of the 6 Power Items that will match with the stat you want to increase during EV Training.

Step6: Battle ingame Pokemon (non link Battle) that gives the EV Points you're planing to increase with the item you chose.
EV Point Pokemon List
Battle 25 Pokemon with the Power item, and 1 Pokemon with out it.
That's maxing out 1 stat.
and battle 3 more of another type, to fill in what EV space is leftover.

There's only enough room to hold a curtain amount of EVs, so you can only max out 2 stats, choose wisely.

HP: Bidoof, Shellos] PowerWeight+ PkRS
Atk:Machop, PowerBracer+PkRS
Def: Geodude, Onix] PowerBelt+PkRS
Sp.Atk: Gastly] PowerLens+PkRS
Sp.Def: Tentacool] PowerBand+PkRS
Speed:Zubat] PowerAnklet+PkRS

Every Pokemon gives a EV point or 2 but these are the ones I'm used to battling

After you're done with the EV's, go to the Sunnyshore Market (not the pokemart).
Speak to the woman to your left, she will give your Pokemon an EffortRibbon to confirm that your Pokemon's stats no longer needs to be EV trained.

If you accidentally gave a Pokemon a EV Point you don't want it to have
There are special Berries that will help you undo that mistake.
PomegBerry-Lowers HP
KelpsyBerry-Lowers Atk
QualotBerry-Lowers Def
HondewBerry-Lowers Sp.Atk
GrepaBerry-Lower Sp.Def
TamatoBerry-Lowers Speed

So don't ignore or discard these Berries, they are here for a very important reason as you can see.