I just went through this process myself. Drawing from your imagination takes taking the rules that a book like "Drawing from the right side of your brain" shows

I just went through this process myself. Drawing from your imagination takes taking the rules that a book like "Drawing from the right side of your brain" shows you and basically practicing. For example, once you learn how to draw the basic human face you can take the memory you have of drawing that face and tweak it a bit. You tweak it until you have something more monstrous, like an ogre for example. That is why most of the stuff that we see drawn from a persons imagination still has very recognizable human components to it. It takes a lot of practice but books these and other lessons you will find are meant to be a building block so that you can draw from your imagination.