Catch all Pokemon in the National Pokedex then speak with the Game Freak director in Castelia City. The diploma will be sent to your home. You do not have to catch Mew, Celebi, Jirachi, Deoxys, Manaphy, Phione, Shaymin, Darkrai, Arceus, Victini, Keldeo, Meloetta, or Genesect.