The Duke and Duchess of Cambridge's daughter will be baptised at St Mary Magdalene church on the Sandringham estate at 16:30 GMT on Sunday.
The service is private but "large numbers" of fans are expected.
Norfolk Police has advised people not to arrive before 15:00 GMT.
Anyone arriving early has been told they will have to queue "in warm temperatures with limited refreshments and facilities" to enter a paddock next to the church.
The paddock's gates will not be opened until 15:00 GMT.
Norfolk Police has advised royal well-wishers to bring sunscreen, hats and drinks - but are asked to "please avoid bringing large bags or coolers".
Parking will be provided on car parks on the Sandringham estate,.
The Duke and Duchess of Cambridge are currently living at the estate.
Well-wishers planning to go to Princess Charlotte's christening have been warned by Norfolk Police they face a long wait "in the heat with minimal facilities in the area".