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.
As AP Alistar, you are going want to always start at the blue buff. Most high-elo players recommend to always start bot lane, but Alistar doesn't have a strong enough clear without the blue buff. After blue buff, clear your wolves and look for a gank. Once you have gotten your runic echoes, make sure to kite the camps in order to recharge your runic echoes.
When ganking as AP Alistar, try to engage on the squishiest target and either W-Q or flash Q then W backward into your teammate. A flash engage is the best way to secure a kill on a target.
As AP Alistar you aren't as tanky as playing him as a support so you need to make sure you never waste your ultimate. You want to dive into the back line, focusing either the midlaner or the AD Carry.