Well you don't "study" for Maths the same way you would for English or History do you? Knowing the difference is already part of that i would assume. Figuring out how to study for maths or history [and how the one requires alot more comprehension rather than memorizing facts] . So saying "you just learn" is as vague as saying "the sky is blue" .
The way you study for Maths/English or whatever is also not the same for everybody.
For example, try asking a group of pupils how to spell a very difficult word...some close their eyes and whisper to themselves, some appear to be writing with an invisible pen, some hunt around for paper so they can write it down with a real pen . You see? Some people hear the spelling, some see it, and some feel it.
If the process of learning and remembering things is so diverse and unique to each individual person how exactly do you teach "study skills"?