If you have 2 level 25 pets and any level 1 pet, obviously start with him in your lineup.

Defeat all eastern kingdoms pet trainers in order of level (don't know exact order, but just do this).

This should get you to around 15-17.

Go to dragonblight and kill the vulture looking things that hang out on skeletons around the tower.

This should get you to around 22-23.

Kill anything in Pandaria or Draenor, if you kill more and more monsters in world of warcraft,you can get more wow gold,this is power your game,or do the daily trainer in your garrison.

Win. I stopped doing this because of time restrictions due to work and RL stuff, but this was getting me a new level 25 battle pet in about an hour, every day. (since the trainers I faceroll are daily)

WoW Pet Battles are a new type of turn-based mini-game within World of Warcraft. Pet Battles allow players to raise their companions’ level (up to level 25), capture wild pets, and even battle other players' companions.

The level requirement to receive Battle Pet Training is level 5, and once you've learned how to battle on one character, all your other characters will also receive the training.