TL;DR: Learn by drawing from real life, so you can more convincingly, creatively and beautifully draw from your imagination (and/or from life).
Beginner artists often obsess about being unable to draw well from imagination. If you try to improve your art skills only by drawing from imagination, it may take you in some very interesting (or possibly frustrating) directions. If you want to draw realistically from your imagination, you should probably start your journey with many many hours of drawing from real life (and to a lesser degree photos and other resources.)
Artists who are searching for advice, sometimes regard the suggestions "pract