this is a tricky question since there are different things about shading i could cover individually haha. but for this reply i'll just go with how i think when i approach shading placement. this might be useful for some, but also entirely unhelpful for those who have issues imagining things (i'm sorry LOL)

light source of course is the first thing that you should keep in mind. which direction is the light coming from? i usually go for somewhere directly above the head if i'm making, like, a character design. just a dude posing.

other than learning technical skills, adjusting my way of thinking when it comes to art helps my brain re-wire itself so i have an easier time knowing how to move forward. for example. instead of staring at my screen and being hyper aware that I Am Drawing, i think about the body of who/what it is i'm looking to make.

i know objects being in the way of light end up blocking it out. if i stick my hand out under my desk lamp, the top of it will be bright and my palm (facing down) will be dark. it's preventing light from getting beneath it, to an extent.

this is basic logic. but i explicitly keep this in mind and imagine what i want to draw as a 3d object, mentally hunting for "things that stick out" as i draw. my process from then on turns into bumping into little run-ins like "oh, the nose is protruding. alright, no light underneath that, then." so on, so forth. i essentially do the exact same thing someone would do if they had one of those anatomy models on hand, except in my head. i draw. stare. continue drawing. stare some more...

if this still gets too hard, i'd highly recommend using yourself and a lamp as a reference if you're unsure of what to do. use your phone to take pictures for reference. it's super helpful. again, this might not be great advice if you have a hard time imagining things, but it's what works for me.