Now you are getting the hang of it!
Whenever a node is green with a checkmark, it's considered completed.
Whenever a square has a letter on it, that's the current level(s).
Do'nt panic - there is no certain order as to which level should be completed.
Just as long as they are all completed by the end and you know what to do, then that's pretty much there is to know about how these maps works.
Good luck!