In response to receiving a few questions as to how I go about improving - figured I'd may as well put this up. It's a huge question, and I can't answer it in its entirety just yet, but when it's something specific then this is typically how I go about it:
First of all, since it's drawing heads, there's a HUGE amount of study material, reference, and course guides available for improving. You could just sit down and just draw from a bunch of references and still improve; albeit, not in the best of ways. Still, moreso to the point that any drawing, or painting, or even time spent considering and applying things to memory, is better than nothing - so don't get caught being indecisive about it for too long.
I was working on this at the time anyways, since my drawing ability and knowledge about facial structure was woefully lacking. Typically in situations like this, I begin with the most basic construction I can give it, and work my way up, occasionally looking at reference and drawing that out as well. Here, construction was from Andrew Loomis, Bridgman, and Vanderpoel. The references were just faces on the internet. After a day, I begin my next sketching session by drawing out everything that I remember from yesterday, checking to see what I missed, and redrawing it; if you don't commit your studies to memory as soon as you can, you've wasted a lot of potential knowledge. Also, check and see what you've learned by drawing from imagination - it's hard, but very fun and rewarding when they come out alright. As I continue to work and grow more familiar with the subject matter, I begin to do drawings after people who I think have a mastery over the subject; in this case, it was Burt Wesley. This way, I can begin to connect the dots with what he knows, and what I'm learning - like why his line weights shift at certain points, why he creates the shapes he does, etc. etc.
Working like this, I've found that the construction helps commit concepts to memory and allows me to draw whatever at most any angle as a 3D object; The reference helps with practical application and also exhibits what quirks can take place within the actual thing; and drawing after better artists helps me see how they've arranged the information I've gathered, and more, into simple, beautiful drawings.
I've been waiting for some time to put a hold on finished illustrations for the sake of working on my figures and faces, especially from imagination, and I've decided to push the time I have to stop finished illustrations entirely and work on improving my paintings and thought process. With all the positive feedback I've gotten from getting a DD - which I'm still freaking out about, y'all are lovely - I feel like this is a good time to slow down and really focus on the how and why I do the things I do, and how I can improve. The light at the end of the tunnel; I have an idea for a finished painting that I'm really looking forward to getting to.
If any of y'all have any more questions, feel free to ask :] otherwise, thank you so much for the lovely feedback. It means the world to me.
Row 1: Loomis, Bridgman, Burt Wesley, A few from imagination
Row 2: Loomis, Bridgman, Burt Wesley, A few from imagination, A few from reference
Row 3: Mostly imagination, one or two from reference, also Bargue plates
Row 4: All from imagination, or reference