Hiya CS
The big trick to doing this is determining exactly what the accent should sound like to the reader and then determining how to type the words which will make those sounds in the reader's mind. And making sure not to step out of the character by getting the accent sounding wrong in their heads. Get those things wrong and, like UW said, the readers will go bye-bye pretty fast. Get it right and you might even go viral.
Just for fun, here's an example of a pretty good job with a particular accent, and of staying in character...
http://www.teddykgb.org/
HTH