8 ways Trump could offend the Queen when they finally meet

"I always say to people that with members of the Royal Family, you can look but you can't touch." Harrold told ITV.

Trump is infamous for his over-the-top handshakes, but he will have to let Her Majesty take the lead when they greet each other, as protocol dictates that the Queen must offer her hand first for a handshake to take place.

It is also advisable that the president does not shake the 92-year-old's hand too vigorously.

The Queen of the United Kingdom and the other Commonwealth realms does not follow — she leads — something Trump will have to bear in mind.

"If they're going somewhere or having a walk around," Harrold said, "the Queen will actually walk slightly in front or alongside Mr Trump."

NATO entered an emergency session on Thursday after Trump broke diplomatic protocol by reportedly calling the German leader Angela Merkel by her first name.

This won't fly with the Queen.

"When you meet the Queen for the first time, the correct term is to say 'Your Majesty', and then after that, it's 'Ma'am', as in ham, and then when you're leaving her presence you might then repeat 'Your Majesty' to finish the conversation," Harrold said.

"Donald Trump shouldn't sit down ... until the Queen sits down," Harrold said. "The Queen should actually sit down first, and she will offer a chair to Mr Trump as she sits down, and then it's a sign that he can then sit down."

When they're done talking, he said, Trump should watch for when the Queen is rising from her chair, and get up at the same time.

On their state visit, Donald and Melania Trump will take afternoon tea with the Queen — which is another myriad of rules and protocols.

"He's got to remember even though he's president, the member of the Royal Family will be the first to start eating, so he will have to wait until ... the Queen begins eating— which is a very strict rule," Harrold said. "And then when the Queen has finished, that means that the course is over, so it will then be cleared."

He added: "So it's worth him bearing in mind that he's got to be quite a fast eater, and keep an eye on what the Queen's doing at all times."

Let's hope the president isn't expecting any important calls during the visit, as the Queen operates a strict no phones policy.

"Unfortunately for Mr Trump, when he's with Her Majesty the Queen, there's be no phones to be shown," Harrold said.

It's paramount that the Queen and all members of the Royal Family remain politically neutral — therefore Trump is advised to steer clear of any kind of political topics of conversation.

Donald and Melania Trump are not required to bow since they're not British citizens. But if they do decide to bow or curtsy, they should do so correctly.

Bowing should be done from the neck, subtly, not from the waist, Harrold explained. And if the first lady chooses to curtsy, she should do so with her right foot behind the left.