STEP ONE: Develope your avatar. That is, define who you're writing to.
STEP TWO: Write a short story about how you had the same problem they're now having.
STEP THREE: Explain how you tried the same things to solve your problem that they're trying now.
STEP FOUR: Explain that once you found (insert your products MAIN BENEFIT HERE) your luck changed and your problem was solved.
STEP FIVE: start to list your products BENEFITS and how they'll also solve THEIR problems, like they did yours.
STEP SIX: Tell them how to get what you're selling
STEP SEVEN: Add value with offer + bonuses
STEP EIGHT: Go for close by telling again how to get it.
There you go - a simple guideline to write a basic, CONVERTING sales letter.