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Easiest way to EV train, in my opinion is wild battles.
But as opposed to going one by one, because of the new Horde System it's a LOT easier.
Certain Hordes give your pokemon 5 or 10 EVs per battle, which becomes 25 or 30 with the proper power item equipped.

http://www.smogon.com/forums/threads/a-guide-to-ev-training-with-hordes.3490052/

List of the Hordes and what EVs they give.

And Super Training for EVs is actually simple. XD You just pick the pokemon you want for it, click the little button with the soccer ball on it. Pick the proper stat you're looking to train in, and complete the training thing by reaching the amount of points displayed that you need to reach. Which you do by throwing the balls into the basket like goals that appear on the pokemon ballon.
The targetting for your balls is through moving the stylus, and you move with the joystick. YOu don't want to get hit by the ballon's balls though, they deduct points.

Also if you're good with memory, the EV a pokemon yields is typically its highest stat. However its not always the case as there are a tie in stats sometimes i.e. Gigalith as he gives both 1 in Attack and Defense as the two stats are exactly the same.

Further information on what EV Training is, which is what you're doing via ST or Super Training or Special Training (h/e you want to call it) is that you get a grand total of 510 EVs altogether. The maximum amount one stat can get is (now) 252, although it used to be 255.

Every 4 EV Points you get is an equivalent of 1 Stat should the said pokemon be at its maximum level of 100. That means if you have 156 ATK with 0 EVs and you'd gain 4 EV Points in that particular stat, it would become 157. However it is only a general estimate as how much you get overall are determined by the Base Stat total, but don't worry about that.

As I stated beforehand, the maximum any stat can get is 252, which is 63 total stats altogether and two stats can be completely maxed out totalling 502 and leaving 6 points leftover which could be put into a stat for 1 Stat point however that last 2 will not give you anything, it's just there to look pretty.

General cookie cutter competitive trend is to usually completely max out two stats in general and slap that last bit in whatever stat such as speed or that smidgen of HP that can or cannot make the difference (Looking Nightshade/S Toss).

However there is no true way to distribute EVs as some can be rather diverse such as 252 HP and 128 in both defenses for example -- this would be good for a wall that can take both ends of the spectrum in terms of attacking while still boasting a deep HP pool, or Bulk as some "smoggoners' like to call it.

For pokemon that hit hard and fast and meant to dish out damage with speed, its sometimes better to just go the cookie cutter 252/252/4 spread in its ATK, SPD, and that last smidge in its HP for Example for like a pokemon such as Weavile who boasts tremendous speed and a rather strong attack while this would be entirely pointless on a pokemon such as Aegislash as they are better to be slower than other pokemon, more commonly another Aegislash due to how his unique mechanics function in the terms of ability.

Now in the case of EV Training through the ST, it's simple, fun and easy once you get the hang of it all. it is very important if you're not going for a 252/252/4 spread to keep track of EVs by the 4s to avoid waste of points. Thankfully the ST program/minigame does this for you by displaying how many you just earned, then at that point its just a method of tallying and adding. As a reference, ST Training grants you the following based upon level.

Lvl 1 Training grants you 4 EVs, 1 Stat
Lvl 2 " " 8 EVs, 2 Stats
LvL 3 " " 12 EVs, 3 Stats

When you use a special bag known as the Double Up Bag, you will gain 2x the amount of the next ST Regimen than you originally would so the 4 becomes 8 and so on.

Now you probably notice the different bags you get during ST Training, these are pretty useful IF you are going for a very accurate spread. There are special bags such as the swiftness bag that give you perks in ST Training to make it easier but grant you no EVs or benefits in the long run, only the Double Up bag does this but do note it only affects the EVs gained in ST Training, not the ones gained from Bags or Battling.

Now Core Training has the bags labeled the stat and the three levels Small, Medium and Large.

Small Grants 1 EV and takes 10 Taps on the Bottom Screen or 1 Tap every minute if you want to leave it be, so that'd be 10 Minutes but you must be on the ST Screen.
Medium Grants 4 EVs and takes 25 Taps " "
Large Grants 12 EVS and takes 50 Taps " "

So Large gives as much as a Lvl 3 ST Level


Sorry if this was long but I hope this helps out. Any questions or clarity issues, ask, I'm very knowledgeable of EV Training.

Pre-Gen6, EV Training used to revolve around getting Power Items, infecting Pokes with PKRS, and then finding a Route that more-or-less exclusively had 'Mon on it that gave out the kind of EVs I was training in.

For instance, in B/W, Route 1 was inhabited exclusively by Patrats and Lilipups, each of which giving 1 ATK EV. With the appropriate Power Item, that became 5 ATK EVs each, and with PKRS that doubled to 10. 25-ish KO'd Pokes later, my 'Mon was fully trained in ATK.

Now with X/Y, EV training is even simpler with Super Training, but in case shooting soccer balls at parade balloons isn't your thing, Horde Battles are a fast way as well, because there's 5 of each monster in each encounter, which equals 5 times the EVs (and with the PKRS + Macho Brace/Power Items, that adds up really fast). Horde battles are kinda rare, but can be triggered instantly by using Sweet Scent or Honey.

HP - Route 5, Gulpins / Connecting Cave, Whismurs
ATK - Route 14, Bellsprout & Weepinbell
DEF - Route 10, Nosepass / Terminus Cave, Geodude & Durant
SpATK - Frost Cavern, Vanillish & Smoochum
SpDEF - Route 7, Hoppip
SPD - Connecting Cave, Zubat / Route 8, Wingull

(Note that if it's raining, using Sweet Scent/Honey won't work to summon a Horde.)

Happy training!