Debretts has a section on this
The key point for anyone receiving an invitation (technically a command when it comes from the Sovereign) is that it will include comprehensive guidance as to what form the event will take. The aim of the Royal Household is to make people comfortable and ensure they have a good experience. Guests are told exactly when to arrive and given advice about what to wear. Any special request can be accommodated, such as wheelchair access or food allergies.
Once at the event, members of the Royal Household are on hand to guide guests throughout. The net result is that people are far less likely to encounter any awkward moments at a royal event than at some other formal events where the hosts may be less experienced. The key recommendation is to relax, follow instructions and not to drink too much alcohol because of overexcitement or nervousness.