I really like how you do your line work, what you struggle with is understanding shading and value. My advice is, stray away from "shading" and just do blocks of black, that will make your piece come out much stronger.
Try doing some more linear art without any coloring and shading, then copying it on different pieces of paper through tracing and try different stuff. But that might be too much work if all you want from it is to be a doodle.
Hope this help you out, follow my page if you ever need more help in the future.
That is clearly not Jeff The Killer, that is my wife, Martha. It is very insulting that you have portrayed my wife in such a inhuman manner. But, I do think you have portrayed her well. Help me, she is a crazy woman that has trapped me in her basement, with nothing but a computer, a bottle of Purell, and sherbert. Correction, it is ice cream, not sherbert. I have tried many different things that might persuade Martha to let me out, but none have worked. I believe I will, unfortunately, never get out of her basement. I love you.