Determine the accent that you desire and find or make a recording of someone speaking with that accent. If you prefer, you can find a speech language pathologist that specializes in accent modification instead, and simply follow her instructions.
Listen to the recording in entirety.
Then go back to the beginning and listen to a sentence or short section of the recording. Rewind again and this time, speak with the recording.
Rewind to the beginning of that section again and repeat speaking with the recording. Do this at least 20 times. There is no number that is too many.
Repeat Steps 3-4 for every sentence in the recording.
Find or make a new recording if you reach the end and haven't yet mastered the accent you desire.