Here are some hints and tips for writing to The Duke and Duchess of Sussex on the birth of their son Archie Harrison Mountbatten-Windsor
Important fact to take more of: it will 3/4 months I would say if not longer before we see a reply.
Address to use
TRH The Duke and Duchess of Sussex
Clarence House
London
SW1A 1BA
United Kingdom
Some samples of what to say
TRH The Duke and Duchess of Sussex
I would like to offer my congratulations on the birth of your son Archie Harrison and I wish the both of you and your son the best.
Or
TRH The Duke and Duchess of Sussex
Congratulations on the birth of your son Archie Harrison. I wish the both of you the best as you start this journey as new parents to a wonderful baby boy. I wish the new family the best for the future
Or
TRH The Duke and Duchess of Sussex
Congratulations on becoming parents to a wonderful baby boy Archie Harrison. I wish the new family the best for the future and sending you my love at this wonderful time.
Of course you can write what you want these are just samples of what you can say.
You can also mention such facts such as how you watched the photocall and were so happy to see pictures of the new family and how wonderful they were. Or how the photo with the Queen was wonderful and how great it was to see everyone.
These are just some tips and hints you can of course say what ever you like in your letter just remember to be respectful.