You give a Pokemon a Mega Stone that corresponds to their species
Choose the Mega Evolution option while choosing your attack in battle
Your Pokemon will Mega Evolve and attack on the same turn
It will stay Mega for the rest of the battle (supposedly) --- http://25.media.tumblr.com/tumblr_mdtqd6EJTY1qlpahho1_500.jpg My husbando <3 QWILFISH for Smash 4! Also waiting on Qwilfish's Megalution, QWILSHARK!
They hold a megastone for their species. You clickie the command to activate it when you have high friendship (friendship is possible not confirmed) BOOM MEGA --- Black 2 Friend Code [Nikl]: 0433-7110-2756 Official Swoobat of the Pokemon X board
During battle, you press a button at the bottom of the move selection screen, and choose an attack. The Pokemon will go Mega and attack in the same turn. The Pokemon has to be holding a Megastone for the evolution to occur, and it turns back to normal after battle. Each trainer can only Mega Evolve one Pokemon per battle. --- The man who will steal your candles. Official Omoikane of the SMT IV board.
They're not true evolutions, they're more like forms. In battle, if your Pokemon is holding a Mega Stone, they can change into their Mega form. They stay that way until the end of the battle. Not every Pokemon has a Mega form, and each Pokemon has a different kind of Mega Stone. --- "The difference between fiction and reality is that fiction has to make sense." -Tom Clancy