How do you become a 5 star general?

There is more than one way to become a general. The most normal way would be that you start off your career by going to a military academy such as West Point, you then get a military position as an officer, but of much lower rank than general, possibly you would start as a lieutenant. You serve in the military and if you do well, you get promoted (and promotions come faster in times of war, when you would have greater opportunity to perform heroically, or to replace fallen superior officers). After decades of service you could eventually rise to the rank of general. Alternatively, lead a coup d'etat and create your own rank.