Meghan will find it a home away from her Californian home. The charming 18th-century house has a swimming pool and tennis court. Shortly after the Cambridges took over, it was given a £1.5m refit, with a new conservatory and kitchen, along with a tree-planting programme.
Even with Prince George and Princess Charlotte knocking around, there is plenty of room for guests to bring staff and dogs. And Prince William will have no concerns about letting unmarried Harry and Meghan share one of the many bedrooms.
Things are different at Sandringham, where Christmas with the Queen is run along lines laid down by her great-great-grandparents, Queen Victoria and Prince Albert - not to mention to a strict timetable.
Presents are opened on Christmas Eve, in line with the German custom Heiligabend Bescherung. Queen Charlotte introduced the German tradition of the decorated tree in the 18th century, while Prince Albert's friend, Henry Cole, came up with the Christmas card, popularised by the Royal family in 1843.
Meghan can expect to find both in abundance at Sandringham.
Coming from the palm tree-lined streets of sunny California, the flat prairies of Norfolk will seem icy and remote; the wind comes all the way from Siberia.
But don't worry about the central heating, Meghan. For all the Queen's austerity, Sandringham is extremely comfortable. Because it was rebuilt by that podgy pleasure-lover King Edward VII, it's more like a luxury hotel than a draughty country house. No dog hair on the bedspreads.
"The Royal family love Sandringham, as it is relaxed," says Hugo Vickers, a royal biographer. "The Queen has the stud close by and takes a great interest in the foals, and horses. They can walk a short distance across the park to church."
As you enter the house, Meghan, you will walk straight into the saloon - a hall set up like a living room, with a roaring fire, Old Masters on the wall and an "Elizabeth R" cushion on the sofa.
It has been noted, by past visitors, as "the favourite room of the Queen and Prince Philip".
Do be careful, though. One day, the late Earl of Onslow and his wife came to stay at Sandringham. Shown into the saloon by a butler, Lord Onslow started examining the pictures. Alongside the Old Masters, there were some amateurish paintings of corgis.
"Come and have a look at these!" he said to his wife. "Complete rubbish!"
A disembodied voice came from behind the stairs. "Lord Onslow, I presume," said the Queen, who was sitting in the shadows, doing a jigsaw.
Luxury is the order of the day at Sandringham. Gone are the days when guests were presented with a laundry bill on leaving. The stingiest now turn up on Christmas Eve with an empty petrol tank. When they leave, it will have been magically filled up.
Don't even think about paying for anything, Meghan. Just leave a tip for the maid - £20 a night is the going rate. Don't unpack either. Your personal maid will do that. Bear in mind that she will see everything in your luggage. On a trip to Balmoral in 1999, Cherie Blair didn't pack her "contraceptive equipment, out of sheer embarrassment". Leo Blair was the result.
If sleeping at Sandringham, don't expect the grandest bedroom. The Queen and Prince Philip - who sleep in separate boudoirs - have the best.
Prince Charles and the Duchess of Cornwall have the next smartest. The younger children, including George and Charlotte, will be in the hands of nannies for some of the time.In the old days, you would never have been allowed to share a bedroom with Harry.
Indeed, Charles didn't share with Camilla, while the Queen was in residence, for years.These days, things are more lax. You will be given a bedroom together, or adjoining. This allows for the ancient art of "corridor-creeping", whereby single men walked on tiptoes from the bachelor quarters to the faraway bedrooms of unmarried women.
The socialite Lady Diana Cooper used to tell a story about early 20th-century corridor-creeping: "When a guest was shown to a room, the hostess would say, 'Lord Kitchener, if you are frightened in the night, Lady Salisbury's room is opposite."'
Because Sandringham was built in splendid style, you will get an en suite bathroom. A useful tip for Christmas gifts: the Queen likes a bath; Prince Philip prefers a shower.
Now, to the clothes. Don't overdo it. There is a black-tie dinner on Christmas Eve, where you can dial up the Hollywood glam, and at which members of the Royal family wear their most opulent jewellery. But don't think about donning a tiara - yet. Dial it down for Christmas Day, too. Think demure for the service in St Mary Magdalene church.
You haven't been christened or confirmed yet; so don't take communion - although it's OK to receive a blessing at the altar. While you're there, glance at the monuments. They will offer a crash course in Harry's ancestors.
Boxing Day will bring shooting; Sandringham is a temple to bloodsports. As an animal lover, you could stay at home and read. On the other hand, perhaps it is best to go on the shoot and stand with Harry. You don't have to do anything except shout, "Over", if you see a flying pheasant.
Remember that you are breaking new ground, Meghan. The Duchess of Cambridge didn't spend her first Christmas at Sandringham until 2011 - the year she married William. When they got engaged, she remained in Berkshire with the Middletons.
But don't worry. As Hugo Vickers says, "Over the years there have been sad Christmases: 1952, when Queen Mary hardly left her room, and 2001, when the Queen Mother and Princess Margaret arrived in wheelchairs. But, this time, there is the excitement of getting to know the next royal bride.
"Ultimately, you are the cause of joy, Meghan, at a joyous time of the year. Have a very merry Sandringham Christmas.
Harry Mount is the author of How England Made the English (Viking)
