Help Building Nidalee?

The guide i'm reading says Ninja Tabi is a good boot if it fits the match, and Skirmishers and Rhunic Echoes should be your JG item. I double checked everything in it on OP.GG so he knows his stuff. The part i'm having trouble with is the "Good Purchases" section. When would you pick any of these items? They seem good but when would you drop one to get another, only get two, one, etc.