EV training guides can get extremely complex depending on how badly you want to win, but this is what I did for the Battle Frontier in Emerald.
1. Get a good Pokemon that you can EV train.
2. Feed it EV berries until it only has EVs in stats where you want it to have EVs.
3. Feed it vitamins in the right stats until you can't feed it those vitamins anymore.
4. Teach it high PP attacks and give it a macho brace.
5. Defeat a lot of wild Pokemon that give the EVs you want. I used these locations, but there might be better places that I forgot about. https://pokemondb.net/pokebase/20092/
6. Stop trying to raise a stat after you defeat a lot of wild Pokemon that give EVs in that stat and it only goes up by 0-2 when you level up.