Not Updated For Current Season
This is how I play my Janna, I'll add more to this after. On how I play, but for now there's my order. The items after Archangel Staff vary depending on the game.
For me when playing this style of Janna, it's all about getting enough AP to 1shot minions for quick gold. Have enough mana that you don't need to worry about the golem so another on your team can have it. Also, shielding your team when in need and pushing back with your ultimate.