To,
Name [name of the recipient]
Apartment name [name of the residential building or apartment along with the apartment number]
Street address [street name along with street number]
City [name of the city]
Subject [mention the subject which is usually to give birthday wishes]
Date [date on which the letter is being written]
Dear [name of the recipient. If the recipient is related you can mention the relation and if it is a formal birthday letter you have to give the appropriate salutation]
1st paragraph: [in the first paragraph mention that you want to wish the person a very happy birthday and give him greetings]
2nd paragraph: [in the second paragraph mention that what do you want the person to achieve on his birthday or you can advise him how to celebrate his birthday]
3rd paragraph: [in the third paragraph give him blessings and best wishes]
Regards,
Name [name of the sender]
Address [address of the sender]