You learn from history and what other people did well and didn't do well. Leadership is a combination of your own innate abilities combined with self-awareness and feedback. From this, you can apply historical knowledge (understand you may not have Washington's height and strength, but may be a better tactical decision maker).

Read biographies on others:

Washington: A Life

Hamilton

Grant

Fighter Pilot (Robin Olds)

Boyd

Anyone you admire - read about them - find out what was good and what was bad.

Mattis's new book is good: Call Sign Chaos.