Write love letters often and from your heart. Don't use other people's words. Your feeling are what is important. Don't fill it with metaphors and trite expressions. Fill your love letter with you warm feeling.
Keep the writing of your love letter simple. This not the place to show off all the big words that you know. Your ten-dollar-words are best used in the next great novel not a love letter. Don't use vulgar or explicit language in your love letter.
Always use a spell checker to make sure that the words are spelled right. Misspelled words will ruin a love letter.
Your final draft should be in your own hand writing and signed. You should close with an appropriate warm closing that sums up and reinforces the mood of your love letter.
Writing a love letter is just like all those short essays you wrote in school. You need to keep focused and concise. You should pre-write and plan it just like you did you school essay. Love letters are much more pleasant than school essays. The art is in the rewriting. Your whole purpose in writing a love letter is to tell the other person how wonderful they make you feel and tell them how important they are to you.
When you are done, put it away for a while and come back to it later. Then read it and see if it needs to be adjusted. Don't send your love letter right after you write it. Allow some time for reflection about the love letter you have just created. You want quality, not quantity.