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in a nutshell, you start by using a reference.
You should notice the proportions / relations while copying a few drawings, and develop a set of guidelines for construction.
for example, you can usually fit an eye between the two eyes. You do like a million of these proportions, until you can construct an entire face and body in a way that is approximately correct.
Please note proportions are like baby training wheels, if you were good you wouldn't really use them (though you probably should at least know them.), its only because ur beg you think about it constantly. Nothing you draw focusing on proportions is gonna look good. The part when things actually starts to look good is when you draw normally and focus on actually trying to make it look good.
Most people are only taught to use reference, and yes reference is essential, but the ultimate goal of every animu/cartoonist should be to be able to make an entire drawing without using any references, you should be able to draw a cute girl (or whatever thing you're autistic about) from the front, the side, and a 45 at least from memory. While ref is a big part of being a beginner, learning to do without refs is where a lot of magic happens that actually makes you good. It forces you to understand things properly, because you literally can't draw without a ref unless you remember and understand the object enough. Its also practicing your direct practical application of your method, like its actually just very good, thats what I found when I did it anyways. I always draw without refs now constantly.
Don't draw without a reference right away, baby artists need ref milk. If it doesn't feel like you're learning much from the references anymore, its a sign you should start doing no-ref. it sounds daunting but its actually quite fun to learn, i personally love it, its worth being brave for, not something you should sleep on.