Some basic things to remember are:
List your contact information; Name, Address, Email, Phone, Website. Trust me you won’t believe the letters people send out (I know, I'm giving myself a side-eye too)
Don’t bring your ego to the game. This letter is all about making a great first impression (free candy anyone), telling them about how great you are and how your book will be the next “BIG THING!!!” will not help you in selling your story or manuscript.
Don’t make your manuscript’s description into some mystery to be solved only through reading the whole thing. This is the first stage of elimination; if you can’t tell them what the story is here there is a very high chance (try 99.9%) that your manuscript will go in the waste bin (recycle goddarnit). Explaining the gist of your story in a short, engaging paragraph is the whole reason you are sending your letter (unless your writers-block has joined with insomnia and then you can't really be held responsible for what you're doing).
What genre does your story fall into? Pick a few words that elect the genre’s general tone in describing your story. For example, don’t use dreaded if you are trying to explain a comedy. Don't worry, you're only trying to sell your soul so no need to stress.