Received Pronunciation, more commonly known as the Queen's English or the King's English, is a formal way of speaking British English. Received Pronunciation was first used back in the 1500s when British royals decided that their way of speaking was the proper and most correct way. Over the years, this way of speaking became closely associated with British royalty and other noblemen of power and wealth. With some time and practice, anyone can learn Received Pronunciation and speak like an English noble.
Study the Queen's English dialect. Consult reference books and news articles that discuss the various attributes of the accent, such as how "r" sounds can be placed in words that don't contain an "r" (i.e. "really" sounds like "rairly" and "off" sounds like "orf"). Practice saying such words to get a feel for how the words roll off your tongue.
Watch interviews and other videos featuring British royals such as Queen Elizabeth II. Listen intently to how the royal person speaks, how she forms her sentences and how she pronounces certain words. Pause the videos as needed and mimic the person and repeat what she says so that you can sound like her.
Use the royal "one." Many British royals and other speakers of the Queen's English use the word "one" instead of the words "me," "I" or "you" to sound more formal. For example, instead of saying "You're upset, aren't you?," someone speaking the Queen's English would say "One's upset, isn't one?"
Utilize a more classy vocabulary of words. Replace "common" or unrefined words with a more tasteful alternative, such as using the word "cad" instead of "jerk," the word "dame" instead of woman and the word "fiendish" instead of "sneaky" or "tricky." Never use the phrases "By God!" or "Oh my God!" in front of ladies or clergymen. Instead use a phrase such as "By Jove!"
Practice speaking the Queen's English as much as possible. The more often you speak in proper Queen's English the more natural your accent will sound and the easier it will be to speak. Refer back to any of the aforementioned learning tools to aid yourself.