Write your address in the top right hand corner of the page. Give each line of your address its own line on the paper, one beneath the other.
Write the date beneath your address. Generally it is formatted as month, date and year, such as "March 25th, 2011." You can include the day of the week before writing the month if you prefer.
Write "Dear (person's name)" on the left hand side of the page, two lines down from where you have written the address and date. If you are writing to someone formally, you need to include his title before his name. If the letter is not formal, you can simply address the person by his first name.
Write the body of the letter, and organize it so that it reads logically. Start with an introductory paragraph that says you hope the recipient is well, and state the reason that you are writing. There may be no reason in particular, in which case you can say something affectionate about keeping in contact. The content of the letter should read through a natural sequence of discussion topics and come to a natural close.
When you have written all that you want to say, write "yours sincerely" followed by your name at the bottom of the letter, below the main content. For a less formal letter you can write "with love," or any other affectionate ending that you normally use with the recipient, then sign your name at the bottom.
Fold the letter and put it into an envelope addressed to the recipient.