How do D&D gods learn to be gods?

[Willie the Duck]

My first answer would be "very carefully".
In original D&D you had to be level 30-something and have a divine sponsor (which incidentally meant that only Humans could ascend because racial level maximums).
Subsequently you ascended to divinity, where you had to choose a form and a few other things, and then you could start earning XP for your divine levels.

That's not from original D&D. It's from the BECMI* version of the basic/classic line. oD&D and the two basic/classic lines preceding BECMI didn't have specific rules for the situation. oD&D would not have wanted a route to attain godhood. That's the one where they published Gods, Demi-Gods, and Heroes as the example of what you are not supposed to achieve**.
*The red-blue-teal-black-gold boxed sets or Rules Cyclopedia
** Hence Tim Kask's line, "This volume is something else, also: our last attempt to reach the "Monty Hall" DM's. Perhaps now some of the 'giveaway' campaigns will look as foolish as they truly are. This is our last attempt to delineate the absurdity of 40+ level characters. When Odin, the All-Father has only(?) 300 hit points, who can take a 44th level Lord seriously?"

Everything since then, and it basically depends on the setting's cosmology.

Definitely this, though, for the rest of TSR (where it was addressed at all). Don't recall if where it is located in 3e and 4e.