This explains it pretty well.
https://bulbapedia.bulbagarden.net/wiki/Effort_values
how do you know it will have an EV of exactly 252 hp?
This is really the only annoying part. There's no way to get the game to tell you what a given Pokemon's total EVs are, so you have to keep track manually. For example: Make a note every time your Spiritomb defeats a Starly that is has earned one Speed EV. EV training is all about seeking out opponents that will boost the stat you want, while avoiding all the rest. For example, if you don't want your Spiritomb to gain any Speed EVs, do your best to make sure that you don't use it to battle Starlys, or any other Pokemon that awards them upon being defeated.
Once a given Pokemon has 510 EVs total (spread between all their stats), they can't earn anymore, so EV training for that Pokemon is over and at that point you can have it battle whatever you want without a second thought.
Other important things:
A Pokemon holding an Exp Share item will also gain EVs from enemies that your other party members defeat in addition to experience points. Make sure that you're aware of this or you might end up with a Pokemon that never battles gaining a mishmash of EVs that you don't want it to have.
Vitamin items (Calcium, Zinc, Protein, etc) add ten EVs each to the stat that they boost. They cannot, however, add more than 100 EV to any one stat. This is why you might be able to give a single Pokemon ten Proteins (for example), but the eleventh one will always fail to work on it.
If a Pokemon does gain EVs you don't want it to have, you still have options. Some berries can lower a Pokemon's EVs in case you want to re-distribute them to other stats via further training. Check the FAQs and guides for details.
