Please read this fascinating article about what a top Dialect Coach in the UK (Barbara Berkery) has to say about the learning an accent and what the reporter who wrote the article endured in her challenge.
I agree with most of what she says although I am inclined to disagree with where she asks the actors to place the sound. She asks them to place the sound in the mask (one of the resonators) to give it a definite nasal quality. I don’t think you can say that the whole of the General American accent lives in that space. I think it’s much more important to keep your tonal sound which may develop aspects of that quality as you change the vowel sounds.
What I absolutely agree with is her description of playing a character with an accent versus playing the accent. Brilliantly described.