Noob question: How to expand a ROM?

This seems like a bad question, but I can’t find the answer explicitly stated anywhere. Like, to expand a 16MB ROM to a 32MB. I just can’t find it anywhere on the internet, even though several tutorials do it as a step.
Again, sorry if this is my stupidest question yet.

You don’t really do anything other than start writing at the end of the rom. There’s nothing you need to do first in order to ensure you can write there.

All tools worth using will handle this for you automatically, in the manner @Tequila described. If you’re finding guides telling you to do this manually, it’s old stuff referring to the days when you would sooner or later be forced to use a hex editor to make your changes directly. In which case (if for some reason you really want to do this) the best answer is “read the manual for the hex editor you’re using”.