The most important thing to keep in mind when writing a tutorial is your audience. Your intended audience by definition has less knowledge in the area that you are presenting. Unlike a classroom though, a person who is having difficulty with your explanation cannot ask you to slow down or rephrase what may have confused them. No matter how clearly something is written, someone, whether they're tired or not reading closely enough, will at some point have difficulty with something. For this reason it is important to include alternate means, usually visuals that will aide your description of what the reader is to be doing.
A good example of this is this guide by the site IGN. The purpose of the guide is to help those who've beaten the game Uncharted 3 find all of the hidden treasures in the game when they replay. Even someone who decides to find them all on their first play through will know how to play the game as the game itself provides a good tutorial. The guide then focuses on the locations, and for the more difficult treasures, how to get to the locations. Notice however, that each treasure is listed with an image so that the player has in game land marks to help them find the treasure.