The fact this question lands in a book-focused space cracks me up a bit—artists' struggles are universal, I guess. From a writer who also dabbles in terrible sketches for storyboards, the only thing that's ever moved me from 'abomination' to 'recognizably human' is a wooden mannequin. Not digital, just the old-school art store kind you can pose.
It forces you to think in simple shapes and masses before details, which is the core skill. All the fancy software later builds on that. I see folks getting lost in Clip Studio's 3D models before they can block a figure, and the results often look stiff.
For pure accuracy, nothing beats real life. Grab a cheap sketchbook and draw people on the bus, in cafes, anywhere. Anime proportions are exaggerations of reality, not replacements.
My final piece of advice, stolen from an artist friend: trace. Not to pass off as your own, but to train your hand and eye to feel the lines of existing art you admire. You learn flow and rhythm that way.