I'd love to start by saying that i'm not a master of imaginative drawing, however It’s something I’m working on improving. 😃
I've spent my primary life as a student learning to draw from observation and depicting reality as it is in front of me and of course i'd always been extremely satisfied with my observational drawings, but then each time i tried to draw from imagination it looked terribly horrible which was quite frustrating.
So, this is something I’ve been working on over a period of time now and i must've say I’ve improved a lot at it presently.

BTW, I’m not kicking against drawing from observation; I think it’s actually the first step we need to get ourselves acquainted with. If we can’t draw our world, how do we expect to create new ones? Learning to draw from observation will help you draw from imagination. Drawing from observation is vital, but it’s only a portion of what I want to get involved with.

Speaking of this👇

It's one of my collections from last year; thought it wise to share this since it was triggered by imagination and fits in with the topic at hand.
Initially, when i held my ballpoint pen over this white sheet, i had no idea what i was hoping to reproduce from the bunch of ideas hovering about my mind or how to go about it; however when i finally set my mind to it, maintaining the focus, and applying the basic principles of art, it turned out less complicated, more interesting, though optimumly challenging.

To be creative, you would need to engage in the art-making process.
Drawing from imagination appears to be a bit challenging than re-creating reality.
This involves a real creation—you're bringing something no one's ever seen before! However, it seems labourious obtaining a correct affiliation of the image in your mind and the lines created by your hand.

Basic tips
Just like with any exercise, regular practice is the key. Assuming you spend some time studying human anatomy over an atlas, feeling like You've definitely gotten everything, and then a week later you're still unable to make any progress without the atlas; sounds like wasted effort
No matter how obvious something seems after a session of studying, you should be sure to repeat it continuosly, making sure to to extend your effort as you get more independent from the references.

Again, order is crucial. Don’t jump between the sessions nor try to learn them all at once; it doesn't make you perfect any faster—it’s so much harder to find out what you’re doing wrong when you’re doing so many things at the same time.
Get yourself a sketchbook or a folder, date them and make notes on the progress You've made; notes tend to be extremely important.... Again, some things may seem so obvious you will be sure you’d never forget them. Don’t trust your memory—trust the practice.
Studying a subject takes a lot of time, because they may turn out so complex. Take at least some quality time to study something new, and spare some time to test your memory.
This would go a long way in building the mind for creative imaginative drawings.

Thanks lots for reading;
Please upvote, resteem and follow to support.
much love;

@joyart