It'd be a bit hard to find the Pokemon with the EV you wanted to train with at level 1; even your starter is at level 5. You could, however, give the Pokemon you want to EV train an Exp. Share, and battle the Pokemon you need for the specific EV. According to Marriland's EV guide, Pokemon holding the Exp. Share will receive the EV while not participating in battle. This will probably take longer for EV gain, since you've obviously used up the hold item with an Exp. Share, and are not using Power Item/ Macho Brace. You can also battle a really high level Pokemon (that has the right EV) with your Level 1 Pokemon holding the Exp. Share. That way, it'd level up really fast, gain the right EV (albeit not much, since it'll gain about 10-20 levels for every defeated level 50 Pokemon, and it'd only gain the 1-4 EV per Pokemon), and then you can switch to using the EV training Pokemon to battle by itself.
Once the Pokemon you're EV training is at a high enough level, you can send it out to battle and give it a Power Item to boost EV gain. EV training at level 1 is probably best, since the hardest thing with EV training is to keep track of it.
Training at Level 50 is workable as well, but obviously you've already battled Pokemon randomly in order to get up to that level (unless you used rare candies). If so, you need to feed it EV reducing berries; 11 of each type
Berry EVs removed
Pomeg Berry -10 HP EVs
Kelpsy Berry -10 Attack EVs
Qualot Berry -10 Defense EVs
Hondew Berry -10 Sp. Atk EVs
Grepa Berry -10 Sp. Def EVs
Tamato Berry -10 Speed EVs
11 of each, because the 1st one will lower its EV stat to 100, and then the other 10 will take that 100 away. Getting these Berries might be troublesome, since you collect them randomly in Dream World, but you can also buy them off a trainer on Route 5 in B2W2 once every day. She will sell you a random berry and 5 of it.
The one advantage that I can see of training at level 50 is that you can always fight Audinos to get up to that level much quicker (since they give out much more Exp.), and it also gives you an increased chance that your Pokemon might randomly get Pokerus and speed up the EV training process.
Also, just remember that Vitamins give 10 Evs each in a specific stat, so you might as well get 10 of the Vitamins you want (because that's the most one Pokemon will take of 1 type of Vitamin) from the PWT/ Battle Subway/ Shopping Mall 9 and use them, since even with a Power Item, EV training can get tedious. Just as a note: you need to get BP (battle points) in order to buy vitamins at the PWT/ Subway, but you can buy them with regular money at the Shopping Mall. Best way to get the cash is to rebattle people like Cynthia & Hugh daily while holding Amulet Coin, and raid the Abyssal Ruins for all its worth, since selling everything amounts to more than 1.7mil.
So, in conclusion, I would begin training from Level 1, using an Exp. Share. This is of course after you've beaten the game, because in-game wise, EVs don't matter if your level is about 5-6 higher than your opponent's. EV training is solely (to me) for competitive battling.
Sources
Marriland's EV guide
Serebii's daily events list (where the EV berry seller is listed)
