There are many reasons why you might want to anonymously write a letter. Perhaps you would like to confess your love to someone without divulging your identity or, maybe, you have important information to deliver but do not want to be connected with the situation. Whatever your reason, there are a few guidelines you should always follow when trying to keep your identity hidden -- failing to take proper precautions might result in the person guessing your identity, which could cause negative implications.
Craft the letter carefully, being sure to avoid turns of phrase or vocabulary that you normally use. Everyone has a very distinct method of communication, and it is your goal to hide your usual style. For example, if you usually are an eloquent writer, be sure to use slang words and bad grammar to throw the recipient off your trail.
Exercise respect for the law when composing the letter. Never threaten someone or harass someone because this can cause her to take legal action.
Conclude the letter without getting into too much detail -- less definitely is more when writing an anonymous letter. The more you write, the more information you will divulge, which makes it easier for the person to guess who you are. Only include the important facts in your letter.
Send the letter by postal service or leave it somewhere for the person to find. If you choose to send the letter by post, make sure the letter and the address on the envelope is typed -- you don't want someone to recognize your handwriting.
Send the letter in a timely fashion. Anonymous letters usually divulge important, crucial information, and timing is everything. If the letter arrives when it is too late, it can cause more complications for the recipient rather than helping his situation.