The way a lot of writers have been known to create accents is by using phonetics and I have to say I don’t like this as a method. Maybe it’s because my regional accent has been mauled by such attempts, or maybe it’s simply because I had to read an entire book written phonetically while I was university and it took me two weeks (Ridley Walker by Russell Hoban). Whatever the reason, I find phonetics don’t work, they make things harder to read and don’t necessarily convey the kind of accent you intend them to. This doesn’t mean you can’t use them, for a start this is just my personal opinion, not definitive. We simply have to be aware of the potential problems of different techniques.
So, how can we create accents in dialogue? The simplest way is to say Mr F is speaking in a Welsh accent, for example, and the reader will, theoretically, change Mr F’s speech in their head to a Welsh accent, whichever part of Wales they think he’s from. People can say this is too easy, some people might say it’s too simplistic, but let’s face it no matter what linguistic tricks you use your reader is unlikely to come out with the exact same accent in their head as you do unless you give them a few cues, and even then it’s unlikely. The reason is that we all read differently and we all imagine characters differently, so unless you find infallible ways to put restraints on your reader’s imaginations their interpretations of your stories will not be the same as yours.
In this case all the writer can do is lead the reader in the general direction their imagination is taking the story and hope they follow along. If they don’t? It doesn’t matter because the uncertainty of fiction is one of the joys. Who hasn’t discussed a book or some other medium with someone else and come up with completely different ideas? So my main advice when trying to individualism your character’s speech is not to worry, write what you think feels right see where the reader goes with it. This is all you can really do in any aspect of writing.
If we’re not going to try and write by numbers the characters voice what can we do? As previously discussed in Distinction in Dialogue there’s individualising the words and phrases they use. Another method is rhythm of speech. Speech does not have to be grammatical, even if someone picks up one of your stories and points out that your speech is not grammatical. It can be, it can be slightly off, or it can be way off the mark. Perhaps an obvious example would be children versus adults; children seem to have an ability to run on and on in their speech until they run out of breath whereas an adult will, in most cases, automatically punctuate in some way. Right there you have an immediate difference in rhythm of speech and you didn’t even have to try too hard. People put pauses in where they might not be needed, they might add extra phrases, periodically digress in sentences then return to what they were saying before and I’m sure you can think of many other things people do in their speech than I could possibly list.
Now I’ll admit that this is no more infallible than anything else, because the rhythm you imagine might not be the one the reader reads. At this point you might wonder why you should bother at all trying to individualise speech if the reader won’t necessarily read it the way you write it. The answer is that they may not read it exactly as you think they should but in all likelihood they will read Mr F’s speech differently to the way they read Mr G’s speech. The point I’m trying to get across is that the window of interpretation in fiction means that if someone reads your writing and doesn’t come out with the character voices you envisioned it doesn’t matter because it doesn’t mean you were wrong. In many ways a story is a collaboration between reader and writer; the writer paints a picture with words and the reader brings the picture to life in their heads.