EV training works as follows:
You have a total of 510 EVs you can divide over your Pokémon's six stats (HP, Attack, Defense, Sp. Attack, Sp. Defense and Speed). The maximum number of EVs attributed to one stat is 252. Per 4 EVs invested in a stat, it goes up by 1.
Common EV spreads are 252 Speed + 252 Attack/Sp. Attack (depending on the Pokémon's base stat values and your moveset) for offensive Pokémon; 252 in Defense + 252 in Sp. Defense for overall defensive Pokémon; 252 in HP + 252 in Defense/Sp. Defense for specific physical or special walls. It's not too important how you spend the remaining 6 EVs (510 - 252 - 252 = 6), but I usually attribute them to HP, or Speed if HP is already 252.
In Reborn and Rejuvenation, you can see the EVs you invested in which stat on the Summary page 'IVs and EVs', which shows IV values on the left and EV values on the right per stat.
In Rejuvenation, there are EV rooms which help you EV train you Pokémon way faster than normal. They are located at the Pokémon Center in West Gearen, and you require a key card for each individual door; key cards can be acquired at the AP Center in the Chrisola Hotel in West Gearen.
In other games, like Reborn, the most common way to efficiently train EVs is to find a location in which the wild Pokémon have an EV yield which matches the EVs you want your Pokémon to have, and have your Pokémon take our these wild Pokémon, while holding the correct Power Items. The Power Items (https://bulbapedia.bulbagarden.net/wiki/Power_item) are items which give you 8 extra EVs in one particular stat, depending on which Power Item your Pokémon is holding, when defeating any Pokémon.
If your Pokémon already has some EV, but you want to get rid of them because you want to invest them in another stat, you can use Friendship Berries (https://bulbapedia.bulbagarden.net/wiki/Category:Friendship-raising_Berries) to subtract 10 EVs per eaten berry from a particular stat, depending on which Friendship Berry you gave your Pokémon.
I think that's all you need to know. If you have any questions, don't hesitate to ask!