How to EV Train in ORAS?

Well you can find ideal EVs on sites like seribi an bulbapedia. Usually you will want to put 252 EVs in two different stats and the remaining few into one other stat, there are a few exceptions. I personally prefer usuing Super Training to EV train. It isn't necessarily faster than horde battles with stat boosting items, but I find it is easier to keep track of the math this way. Super Training can also produce a Reset Bag that can reset EVs in case you accidentally make a mistake when training. This is useful if you already have a pokemon you bred for IVs and trained up without focusing on EVs.