How do actors?

The answer is
Voice couching. If the actor doesn't already have an accent in the bag from their training or
previous work, there will be a dialect coach hired by the producers to assist, and they'll be present
through the rehearsal process. But a professional coach is not necessary. There are a lot of books with audio which help to develop their accent.

Training accent without a coach
They listen & do the exercises. There are explanations on phonemes, vowel, & phrasing
pronunciation, which were repeated several
times for you to say and match to the recording. Also there are dialogues by natural speakers,
to listen to and practice mimicking. It’s as well
effective as to train accent with a couch.

Looking for a specific accent
When auditioning an actor might be told in the
casting notice what sort of accent they're looking for, and they may even give names of actors or
movies to watch for a guide. And then once an
actor is hired, if a production requires a
specific accent there should be a dialect coach (if budget allows) who will work with all the actors on vowel and consonant placement, intonation,
rhythm, and tempo.

Professional coaching
A lot of it comes down to drilling phonemes and specific sample words. You have to learn
(or relearn) how to shape your mouth to make a
certain sound, then make a new habit of that
placement.

Aspects of accent which must be taught
A diligent actor will mark up their entire script with
the proper pronunciations. There are the aspects of
accent he or she must pay attention to:
The sounds (pronunciation of consonants and vowels)
The rhythm (word and sentence stress)
The music (intonation)
The flow (thought groups, linking, and connected
speech)

Which accent is high-demand?
It is more lucrative for a British (or Australian) actor to learn to do an American accent because
the movie and TV market in the United States is larger than either of those countries, and, by and large, the demand for British and Australian
accents in the US is lower than is the demand for American accents.

Accent coach: how I teach actors
https://www.youtube.com/watch?v=2ak3wQ_yq0U

Top 10 actors who can pull off foreign accents
https://www.youtube.com/watch?v=iLeXB925yww

Which American accent is the hardest for British actors
https://www.youtube.com/watch?v=6E4TOHGO0Ms
There’s more than one