What's the best approach to learn mathematics from the bottom-up?

Let's assume all I have is some shitty highschool-tier math background knowledge (which is also obsolete for 10 years) how do I go on about learning it all?

I don't only want to simply absorb the knowledge in order to read certain papers but also want to learn why stuff is done the way it is.

Is a historical approach to this good? Should I read myself into the history of mathematics to get a deeper understanding of why math is done the way it is? Or is that a waste of time?