How do I go from notebook doodles to something actually half decent?

rhymes with "licky"

you can find it on this board

See bigger. Get better paper, better tools, and then learn the basics, but make sure to approach them in a way that is satisfying to you, or else it can get boring af

>>2483660
Basically start with drawing staples (anatomy, still life, point perspective, figure drawing, speed and action drawing, landscapes, line or contour drawing, cross shading, detail, lighting, etc etc) do some practice work with different types and styles of drawing and techniques then see which you can apply to an existing drawing or a new idea to make it better than the last.
The key is understanding and practice. The only thing that makes a great artist better than a good artist (or doodler) is understanding what to do to improve a work and noticing where that improvement can take place.

Your sketch for example is not bad in terms of anatomy but some areas are off (head chest hands muscle structure) your lines are rough and either imperfect structurally (ie not conforming to reality, gravity, organic curves, perspective shift etc) or visually (straight lines that aren't straight, redrawn sections, guide lines, imprint lines too prominant) and overall the thing that brings you down (possibly in your own eyes more than any) is time. I won't persume to give an exact time it took you to do this piece but I'll wager it wasn't a long time.
I would not call myself a great illustrator but I'll happily spend more than 3 hours on a drawing before I call it finished (that may not be in one sitting). Granted not all drawings take that long but an hour on a good peice (A4 with moderate detail and shading) is quick.
even a throw away doodle will take 15-20 minutes to put to paper.

TL;DR practice some techniques, apply them to pictures to get better, spend more time and effort on your works. Don't be afraid to go back and spend more time making works better after learning more. practice practice practice