So to learn assembly

I need to read the processor manufacturer's manuals?

That's one way to do it, though probably not the wisest.
I learnt x86 assembly by learning how to reverse engineer binaries. Then I began writing trainers in MASM, after
reading some tutorials, and it pretty much took off from
there.

With x86, probably the hardest thing to learn is how
the stack works, and the x86 segmented memory model
(though rather obsolete now due to the 32-bit address
space and virtual memory.)