EVs in Gen V are applied as they're collected. For every 4 EV points in one stat, you'll receive one point in that one stat. ie; If you fight four Pidove, you will have have a point applied to attack, because Pidove gives one Attack EV.
255 is the maximum amount of EVs you can invest in one stat, but you want to cap it at 252 as the extra 3 EV points will count towards nothing. 510 total EV points can be distributed in total, across all stats.
The easiest way to train is by buying the 'Power items' from the Battle Subway. While equipping these, your Pokemon will gain 4 EV points in a specific stat, depending on the item held, PLUS the EVs they would normally obtain from the Pokemon you fight. ie; You fight a Pidove. You obtain 5 total EVs. 4 due to your Power Bracer, and 1 for the Pidove.
But if you can't obtain Power items yet, Macho Brace doubles the EVs you would normally obtain from battle. If you have cash to burn as well, you can buy vitamins and apply them to a Pokemon for a bonus of 10 EVs to whatever stat the item applies to (ie; Protein, Attack. HP Up, Hit Points), up to 100 EVs in one stat. After that, you must train instead of using vitamins.