You don't.
In my campaign settings, no-one can become a god. I've never thought it was an interesting part of the D&D experience.It's a simple question. How does one become a god in your campaign?
In published settings, where it's theoretically possible? I don't expect to ever have to account for it given the people with whom I play, but in Eberron for instance it's theoretically possible for a mortal to aspire to replace one of the Sovereigns . . . at least, if you assume that the dragons who follow the Thir religion are correct . . . so if a friend of mine came to me and said "My character wants to replace Aureon", I'd figure something out.