For new users, it is often extremely frustrating that they cannot seem to align the Rotate or Protractor tools in any, arbitrary direction / angle. Below are some basic tips on how to do this easily. Some of what is written here can be applied for the Circle, Polygon or Rectangle tools as well.
To practice any of the methods below, best is to create a simple model which has some edges/faces that are not aligned to any of the world axes and major planes defined by them. Group your geometry first and pre-select it before starting the Rotate (or in other cases the Move) operation. Of course, for the Protractor tool, you need not pre-select anything.