Listen to Daws Butler Teaches You Dialects by Mr. Daws Butler on Audible. https://www.audible.com/pd?asin=B004YLSCSS&source_code=ASSORAP0511160006&share_location=pdp
This book was the first that I studied for dialects when I started narrating fiction six years ago. I still review it regularly as I am essentially a mimic and listening to Daws Butler for this hour provides good practice for me.
He is funny and I love the explanations he gives like the ideas of tasty words and feeling the words in your mouth.