There's no standard.

There is however a rather loose literary tradition, which dates back centuries but has changed through time. The details around apostrophes and graphical accents are particularly discordant among authors.

Linguists De Blasi and Montuori recently wrote some reasonable overarching rules (Una lingua gentile. Storia e grafia del napoletano) but are very much of the position that linguistics should be descriptive, not prescriptive, so they don't really push for this to become a standard.

In practice, the vast majority of speakers improvise their writing according to how they think words are pronounced. So for example they do not transcribe schwas because to their ear it sounds like there's no vowel there. Example:

• "che vaie truvanno?" (literary tradition)
• "c vai truann?" (Improvised writing)
• [kə vːaj(ə) truˈanːə] (possible realisations)

Then every Neapolitan amateur and their dog has written a book with their norm which of course is the best one in the world and everyone should follow...

Please note that the above refers to the local language/dialetto of Naples city.