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.
Basically just follow the skill set and for the items, get them in order of what i have down then after you get the Void Staff, just fill the other 2 slots with another Zhonya's Ring. Late game sell the doran's ring and get another Zhonya's ring. This build is only for AP and it does make u a little squishy at the start of game, but make sure ur solo or try to get fed.