Approach an anonymous letter as if writing a normal letter. The only big difference is that you will not include your name or any other personal details. Otherwise, the basic layout and structure of the letter are the same as normal. This means that the content of your anonymous letter is dependent on what you want to write about, to whom and why.
The first tip for writing an anonymous letter is not to write it. This does not mean giving up on the whole venture, but it does mean typing it instead. It may seem paranoid to not hand-write the letter, but typing removes the possibility of your handwriting being spotted. Use a typewriter or a computer instead. If the letter is extremely sensitive in nature, regarding information you do not want traced back to you, then consider using generic paper and printers in a print shop rather than your home printer.
Include the address of your target in the letter, but exclude yours. Keep the top-right area of the letter blank. Instead, begin the letter with the date on the top-left and then your target's address immediately below it. Address the letter as normal with "Dear sir/madam," "To whom it may concern," "Dear Title" or "Dear Joe Bogs."
Think carefully about the content of the anonymous letter before you write it. What do you want to say and why? Hone in on the details and the arguments and do not let yourself get distracted. Remember to thoroughly edit your letter and fact check wherever possible. You do not want your arguments to be ignored because of basic errors.
There are many legitimate reasons for writing an anonymous letter. Basic examples include letters of complaint, such as to a restaurant for their bad service, or letters to a writer whose opinions you do not agree with. Many times, people who write to advice columns choose to write anonymously due to the potentially embarrassing nature of their question or problem.
Two important types of letters that are usually anonymous are the whistleblower and those reporting people for something. With these two examples, it is especially important to get all of your facts correct and in place. It can also be very important to not reveal your identity, as doing so often has consequences.
Once finished, sign the letter however you wish; this could be a simple "anonymous" or "a concerned employee" or something similar. Bear in mind that when mailing the letter, some recipients will do their best to work out who sent it. You may want to mail the letter from a different post code or while on a business or pleasure trip away from home or email the letter from a generic mail service with a new email address set up just for that one email.