Invitations are likely to be dispatched soon to some 800 guests.
US president Donald Trump is not expecting a coveted invite to arrive in the post, but some of those who might be include former American leader Barack Obama, tennis star Serena Williams and Ms Markle’s Suits co-stars Sarah Rafferty, Patrick J Adams and Wendell Pierce.
In 2011, Prince William and Kate Middleton’s invites – traditional white cards, gilded along their edge with gold, with the royal cypher – were sent out around 10 weeks before their wedding.
Harry and Ms Markle have around 14 weeks until their big day.
They may opt for the traditional royal invitations on thick white card, which begin with the words “The Lord Chamberlain is commanded by The Queen to invite …”.
But Ms Markle is an expert calligrapher, who used to craft invitations for celebrity weddings, so she may decide on her own alternative style.
The Lord Chamberlain’s Office at Buckingham Palace is responsible for organising royal weddings, but Harry and Ms Markle will be leading the show.
Under the guidance of Harry’s private secretary, Edward Lane Fox, the Lord Chamberlain’s Office will deal with the ceremonial aspects of the day.
The best man, page boy and bridesmaids
The Duke of Cambridge is expected to be Harry’s best man, or perhaps jointly take on the task with one of Harry’s childhood friends.
William has already joked that he is yet to be asked by his younger brother.
Prince George and Princess Charlotte look set to be page boy and flower girl.
But Ms Markle may also choose some grown-up bridesmaids from her close set of friends – most likely Canadian stylist Jessica Mulroney and Indian actress and philanthropist Priyanka Chopra.
Ms Markle could even pick a best woman.
Christening and Confirmation
Ms Markle is going to be both baptised and confirmed in the Church of England ahead of her religious wedding.
She can choose to be fully immersed in water in a special pool at her christening, but it is more likely she will opt for a small trickle of water being poured on her head at a font.
The baptism and confirmation, expected to take place privately, can happen on the same day.
Shortly before her wedding, Kate was confirmed in a secret service at the Chapel Royal in front of William and her family.
Harry and his bride-to-be will also be deciding on their chosen music, hymns and prayers for the traditional ceremony, which is expected to be led by the Archbishop of Canterbury.
The couple will also have to settle on their vows, but Miss Markle, who was a UN women’s advocate, is unlikely to opt to obey the prince.
They will probably choose the Series One (1966) Book of Common Prayer ceremony, just as William and Kate did, which allows the bride to drop “obey him” and “serve him” from the religious proceedings.
Harry also has to decide whether to wear a wedding ring. William does not wear one.
Ms Markle’s stylist and close friend Jessica Mulroney runs a bridal store in Toronto so is said to have been helping the bride choose her gown.
Bookmakers Paddy Power have one of the Queen’s favourite designers Stewart Parvin at evens to create the wedding dress, followed by Roland Mouret at 2-1, Erdem at 6-1 and Victoria Beckham at 7-1.
Peonies are one of Ms Markle’s favourite flowers, so it is likely her floral bouquet will include these fragrant blooms, which are in season in May.
As is tradition for royal brides, there is also expected to be a sprig of myrtle, grown from the original myrtle in Queen Victoria’s wedding bouquet.
From the food to the cake and the seating plan, there is much to decide on for the wedding breakfast – the meal following the ceremony – and the evening celebrations.
Harry, formerly known as the Party Prince for his outgoing ways and exuberant socialising, will no doubt already have plenty of ideas to hand.
The prince will be putting pen to paper to write his speech for their reception.
But Ms Markle, who is an accomplished public speaker, is expected to break with tradition and deliver her own as well.
Sir Elton John has reportedly cancelled two of the dates of his Las Vegas residency to attend the royal wedding, amid speculation he could also be performing.
Ed Sheeran, Sam Smith and grime artist Stormzy have also all volunteered, but no acts have yet been confirmed.
Plans will already be in place for policing the grand royal occasion, with thousands of people expected to descend on the town of Windsor in Berkshire to catch a glimpse of the bride and groom.
The taxpayer picks up the large bill for security at the event.
Souvenirs and official titles
The royal bride and groom will also have to approve Royal Collection wedding souvenirs, decide on a design for their cyphers and look at their Conjugal Coat of Arms.
Harry will also have a discussion with his grandmother the Queen about the couple’s future titles – most likely the Duke and Duchess of Sussex.
The prince and Ms Markle will be hoping their chosen destination stays top secret as they enjoy a break after the busy run-up to the wedding.
Botswana, where they holidayed in the early days of their whirlwind relationship, is a good bet, but wherever they go, the newlyweds will be looking for seclusion away from the limelight as they start their married life together.