Oh but come on! Can't you google it ?
We will take an example: Growlithe. Say, I wanted to EV train Growlithe and make it a strong Arcanine. First of all, I'll be trying to get good IVs. For this, go Google. I myself usually hatch an egg, but this is none close to mandatory, except if you want breed moves, as Close Combat Arcanine. You go on Serebii, and search for the best place to EV train, or easier, this question.
Afterward, you plan a spread. For example, I want 252 Atk / 252 Speed / 4 HP.
For this, I must go, if I have low leveled Pokémon as this Growlithe, to the Flocessy Town where you find Patrat, Riolu, Pidove and Lillipup which all give 1 Attack EV out of 252. Now, you must have power items. They are in the PWT shop or the Battle Subway. Here are more details. I give my Growlithe the Power Bracer, for the attack, and thus each time I defeat a Pokémon who give attack EVs, I gain 5 Attack EVs out of 252. I may battle over and over until I have reached 252 EVs in attack. My Growlithe must be strong enough to battle Basculins in Route 19: here for more details. For the HP, I battle 2 Audinos. And you do this for each stat. The Special Attack is the hardest, as you may need a high level to go battle against Litwicks in the Celestial Tower.
Is it clearer ?