AP Kog'maw

I just started learning how to play Kog. I have two games on him, both ap, and overall both went well for me. The first game was kind of a bust, but the second game was a slaughter. The only other time I think I have done so well on a new champion is the first time I played nidalee. Anyway the only problem I have is I don't know what order is typically best to buy certain items. My first full item is ludens. I know I get this first, but after that I don't really know what to get. In the second game I got Rylia's second and archangels after that, but by the time I had the gold to get archangels (30 minutes in) my tear had been stacked for quite a while. After I got those I got liandries and finally void staff (the enemy team was building no mr so I saved void for last). I'm not really sure what I should buy other than rylias, ludens, and archangels, but I don't even know what order to get these in other than ludens first. So if you know what I should buy and when please let me know.

Here is game 2 so you can see my build path if you want it --------> http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1981344033/214906406?tab=overview