Write your letter. Get creative with it if you want, by using a different colored pen, buying special stationary or even using stickers and glitter.
Include a picture of J.K. Rowling or a removable cover from one of your Harry Potter books and a self-addressed stamped envelope or International Reply Coupons (available at the Post Office) with your message, if you want an autograph sent back to you. Make sure your picture or book cover isn't folded or torn in the mail by putting a stiff piece of cardboard in with it and writing "Do Not Bend" on the envelope.
Send your package to one of two addresses: J.K. Rowling, Christopher Little Literary Agency, Eel Brooks Studios, 125 Moore Park Road, London SW6 4PS, UK or J.K. Rowling, Arthur A. Levine Books, 557 Broadway, New York, NY 10012. Make sure you write clearly, so that your package arrives safely.
Wait patiently for a reply. This popular author likely gets hundreds of pieces of fan mail every day, so it is important to be patient for at least three months when waiting for your autograph. If you don't request an autograph, it is likely that you won't get a response at all.