How to Mega Evolve a Pokémon in Pokémon X and Y

Get the Mega Ring. You get the Mega Ring after beating the Shalour City Gym, going to the top of the Tower of Mastery, and beating Korrina in a battle.

Get the appropriate Mega Stone for the Pokémon you want to Mega Evolve.

Have the Pokémon you want to Mega Evolve hold the appropriate Mega Stone.

Go into a Pokémon battle. Once you have done so, tap the "In Battle" button of the Pokémon you want to Mega Evolve on the bottom screen, and tap the "Mega Evolution" button.

After you've selected an attack, your Pokémon will Mega Evolve before using the attack. Sometimes your Pokémon will change its type and ability.