A Buckingham Palace spokesperson said: "Princess Eugenie and Mr Brooksbank have taken great care and interest in bringing together the content of their service, working closely with the Dean of Windsor and all others involved".
The nuptials come just five months after the glittering wedding of Prince Harry, younger son of heir-to-the-throne Prince Charles, and USA actress wife Meghan, now the Duchess of Sussex, at the same location.
When Princess Eugenie walks up the aisle on Friday, she will be accompanied by six bridesmaids.
The pair have also given a final pre-wedding interview to ITV - the channel broadcasting the event live in the United Kingdom - detailing the first time they met and their impressions of one another.
It will mark the first time little Theodora has been seen at a public event as X Factor judges Robbie and Ayda have always chosen to keep their children out of the spotlight.
Another 1,200 members of the public selected by ballot will listen to a live broadcast in the castle grounds. She has lived most of her life outside the media spotlight and keeps a low profile compared to cousins Prince William and Prince Harry and their glamorous wives.
But a spokeswoman for the council said that in terms of crowd numbers and associated stewarding and policing costs, it expected Harry's and Eugenie's nuptials to be "two very different weddings on two very different scales".
Eugenie's sister, Princess Beatrice, is maid of honour.
Eugenie has worked for several years in a fulltime position with the Hauser and Wirth art gallery in London. He and Eugenie have dated for many years.
The Queen will then host a reception for the couple and their guests at the castle.
Well known faces predicted to attend the second royal wedding of the year include singer Ellie Goulding and fiance Caspar Jopling, supermodel Cindy Crawford, David and Victoria Beckham, Harry's exes Cressida Bonas and Chelsy Davy, model Cara Delevingne, singer James Blunt and wife Sofia Wellesley, and George and Amal Clooney.
Criticism has already been levied at the estimated £2 million security bill, which falls to the taxpayer, for Eugenie's wedding.
"We are getting a lot of American and Chinese and Spanish visitors", said Jag Khaira at the Nell Gwynn Tearoom.