Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
When your buying your stuff, there 2 ways:
Buy Doran's Ring and continue until you have Mejai's Soulstealer. You can either buy Rabadon's Deathcap first or Archangel's Staff (Depends on how much money you have and if you want/need more ap for dmg on the Hextech Micro-Rockets) and then continue until you have all you item slots full, then sell yur Doran's Ring and buy Morello's Evil tome.