Read her books and watch her TV shows and specials. Be familiar with her work and material. Don't write a fan letter if you do not know what she does or has done. Knowing her work will allow you to put together a thoughtful letter.
Write something exceptional. Do not write a letter saying you think she is beautiful and funny or make any other generic comments. Write a letter that will touch her heart rather than make her set it in a pile with all the other letters.
Be concise. Don't mail her a novel. On the other hand, do not send a letter that contains only a few sentences. Write a well-written paragraph that opens with, "Dear Chelsea," and closes with, "Sincerely," and your name.
Don't ask for too much. A lot of people ask for signed photographs and other memorabilia. Chelsea Handler is a busy person, and she is most likely not the one who will be reading your letters. If you would like her to send you something, be polite and be understanding that she may be unable to meet your request.
Mail your letter to: Chelsea Handler, 2118 Wilshire Blvd., Suite 1053, Santa Monica, CA, 90403. Include a return mailing address so she can reply to you.