The Duchess of Cambridge married into the Royal Family and has barely put a foot wrong since becoming an HRH.
Here are some nuggets of advice that Kate might offer her future sister-in-law as she prepares to embark on a life with the Windsors.
Get to know Granny
The Queen, with her many years of experience, has much to teach royal newbies.
She's also “The Boss” - what she says goes. This could be everything from the uniform Harry must wear on his wedding day and how royal children should behave on the Buckingham Palace balcony to etiquette for official engagements.
As the Duke of Cambridge once said: “You don't mess with your grandmother.”
Watch out for the long lenses
Kate was snapped sunbathing topless by photographers while enjoying a break in the south of France in 2012.
William was furious at the intrusion and a costly legal fight and court case ensued, with William and Kate being awarded more than 100,000 euros in damages.
Make sure you have a trusted circle of friends and staff
Discretion is key for William and Kate. Within Kate's close circle of confidantes, long-term friend Sophie Carter was rewarded for her loyalty by being asked to be Princess Charlotte's godmother. Ms Markle will need to be able to rely on friends not to blab to the media about her life with Harry.
Staff are also vital for ensuring the smooth running of royal life. William and Kate have a nanny, a housekeeper, an orderly, as well as their bodyguards. Kate also has a private secretary, a PA and stylist, and a press team, and when she is on tour, her hairdresser goes too.
Get a house in the country
Anmer Hall, a wedding gift from the Queen, has given Kate, William and their children, sanctuary and privacy away from London. The 10-bedroom country retreat, which is a short drive from Sandringham House in Norfolk, underwent a major refurbishment overseen by the Cambridges.
Do not worry about buying your own tiara
The Queen often loans priceless jewellery to family members, meaning there is no need for Ms Markle to head to Cartier to begin her own collection. Kate borrowed one from the monarch for her wedding.
Focus on your interests
Kate has been careful to support charities which reflect her passions - such the arts, tackling bullying and improving the mental health of children.
She is likely to guide Ms Markle into doing research and taking her time before picking patronages - many of which she will be committing herself to for decades.
Protect your children at all costs
For William and Kate, protecting their children from the limelight and intrusion is paramount.
In 2015, Kensington Palace accused paparazzi photographers of harassing Prince George, saying the royal youngster had become their “number one target” and accusing them of going to “extreme lengths” to get pictures of him.
George and Charlotte are rarely seen in public, except on overseas tours and at the occasional UK appearance, and the Duke and Duchess release a number of photos throughout the year in an attempt to keep the public and press happy.
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