What would a genius instruct his child self to study, within mathematics, in order to most rapidly accelerate his learning?

(Obviously, assume time travel is possible.)

In my brief sojournes on this board I have found not a single comprehensive guide for self-instruction on mathematics. There is, however, a suggested book list containing tens of thousands of pages of material. Highly impractical.

For comparison, when I first began playing Starcraft, I found dozens of high-quality guides which were (when viewed relative to each other) able to very generally sketch the outline of "how to play."

Where is the "how to play" guide for mathematics?

Assume passion. To those accomplished,

(1) "What would you tell yourself to study were you a novice to the field?"

(2) "How would you instruct yourself to study?"

(3) "Which field would you recommend to your former-self?"

(4) "Which branch of mathematics is the most universal? That is, which branch is not only so widely accepted as self-evident but is also as practical as it is difficult?"

(5) "Which book on mathematics is your favorite?" (biography, text, or any genre)

Thank you for your time.