well, the way i do it. is hatch a pokemon from an egg. or catch it in the wild. normally ill make sure i have the nature i want before i will EV train. ill also hatch or catch in another one of my pokemon games so i get the EXP bonus.
so for example, i will hatch an adamant natured gible.
and what i will do is because i have a pokemon with pokerus, ill put gible in the second part of my party. the pokerus infected pokemon in the first slot.
make a couple wild pokemon faint. i will repeat this until gible has caught the infection, this never takes more than 10 battles for me. normally from 3 - 5
after gible has caught it, i will put it first in my party. and switch it out for a stronger pokemon to defeat a wild pokemon. this way gible gets the exp for the battle but won't get any EV points.
i will repeat this until i am confident gible can handle making in this case Goldeen faint by itself. at this point i will attach an EV boosting item.
so golden gives +1 EV in attack. plus the bonus for pokerus and the EV booster item.
now, because i don't want to add up to a perfect number how many goldeen i have to kill, ill just normally make them faint all day. but on average. with all the things this gible has. (the one were talking about in example) it would only have to make 150 - 200 goldeen's faint for max EV in attack. you will have maybe 2 - 5 EV's left over to use on another skill. which has to be done because I'm 99% sure that all ev's cannot be put into one skill.
you might get a couple big bonus for gibles attack at the start. but after a while it will go back to normal. what i mean by this is you might get a +10 in attack for a level, but for the next levels you may only ge +1 - 4 don't let this worry you. you have still done your part.
i hope i haven't confused you. you can google which pokemon give the EV points.
for example just search "attack EV pokemon" or "defence EV pokemon" in google. and it will give the information you need.
if you have pokemon you have already trained but wish to EV train them properly. just search "EV reducing berries pokemon"
that will give you the information you need.
now, just say you didn't have pokerus or a ev booster. you would probably have to kill 300 - 500 pokemon for max EV.
it sounds like a lot. but if you work at it all day. you will get through it. besides, you'll get loads of exp for your pokemon even if the battles are from weaker pokemon.
if you have any other questions. VM me. hope i haven't confused you.