1. use the walkthrough 1.9 mega linked in op - page 1
2. you can of course play without the walkthrough, but it helps to look at it every now and then since there are several hidden scenes and prerequisites choices that Lily has to make to trigger the various paths and future scenes. Choices matter and if you make the wrong choice once you can miss a lot. Besides, it's also fun to try different paths to see different scenes - it's not always obvious which one is the best path. The guide, while not perfect (and out dated), will help you make better informed choices.
The step from v1.9 to 2.0 does not really require a guide so walkthrough v1.9 will do. hope that helps.