From that ideal neutral, each dialect in the world changes the sounds of letters a specific way. Using the phonetic alphabet, actors can teach themselves a specific accent. Terrific, right? It brings up a couple of problems especially for writers.
- Accents are meant to be heard, not read - in a play, a dialect is music. In the written word, it's nails on a chalkboard. See James's H.P. Lovecraft example if you need proof.
- For actors AND for writers, the phonetic alphabet only tells you how word sounds change - think aboot rather than about. There's a whole other aspect to dialect that actors cannot actually write into their scripts unless they have music training. It's a couple of concepts called cadence and tonality.
Here's the point I took so long to get to: All of us know what an accent is. All of us, unless we've been living under a rock with no internet connection, have heard all kinds of dialect. Southern, Bronx, Aussie, British, Irish, Scottish, Slavic, Asian, you name it, we've all probably heard it. As an actor, my job is to reproduce that accent so well that a native will mistake me for a fellow countryman. Your job as an author is the exact opposite.
There's a story about an interview with Lawrence Olivier wherein he said it wasn't his job as an actor to feel anything. It was his job to make YOU feel everything. Perfect description, I think, of writing dialect. An author's job isn't to dictate what she hears in her head to her readers - it's to suggest just enough dialect that the readers hear dialect for themselves.
The best tool for hearing dialect? Movies, TV and Youtube. Americans have a tremendous resource in the BBC because there you have native speakers from all over the world. You can hear the music, the word choices, how sounds change. Youtube has videos from people all over the planet and you have immediate access to hearing speech patterns from just about anywhere. Where this falls down is in historical contexts because every historical period had its own dialect. Shakespeare, obviously, but also very notably the Regency...but that's another issue entirely. If you care about that, look at plays from the period. The speech patterns are very different from what we're accustomed to.
However. This stuff is research and backstory. It needs to be done and then it needs to be nearly invisible. Hint at these speech patterns in writing - preferably with word choice, regionalisms, and intonation rather than with dropping every g or all the r's in someone's dialog. Otherwise, you aren't trusting your reader.