Here's how to EV train the quickest way possible.
1) Optional, but helpful: Get the Pokemon infected with Pokerus
2) Decide what stat you want to max out
3) Get a Power item (Power Bracer, Power Anklet), or the Macho Brace and make the Pokemon hold it
4) Use 10 Vitmanins related to that stat)
5) You get 100 EVs from the vitamins, the Pokerus doubles EVs from battles, Power items give 4 additional EVs, Macho Brace doubels EVs like Pokerus does
6) Battle the Pokemon for the right EVs
Good wild Pokemon to EV train with
Attack: Bibarel, Defense: Steelix, Onix, Graveler, HP: Marril, Sp Atk: Roselia, Sp Defense: Tentacruel, Speed: Staravia,
Look the Pokemon up on Bulbapedia to see how many EVs each give out in that stat, and be sure to take into account the effects of Pokerus, Power items, or Macho Brace. Also not that Exp Share DOES NOT split EVs so if you knock out a Pokemon worth 2 HP EVs bothe the battled and the Pokemon holding Exp Share both get 2 HP EVs each.
Advice: A Pokemon can have a maximum of 510 EVs total and 255 EVs max in a single stat BUT since only numbers divisble by 4 are relevant, 252 EVs is considered maxed out.
You also need to pay attention to Natures, which also affect your stats, each nature will give a 10% boost in one stat and a 10% drop in another. Here's a chart:
https://www.gamefaqs.com/portable/ds/file/925601/48715
IV breeding is a pain. Most people use
https://shoddybattle.com/ to battle competitively so they can avoid the hassle of EV training and IV breeding.
For ideas on how EV train your Pokemon and what moves, items and natures to use visit
https://www.smogon.com/