Bella Dia has a fantastic tutorial on how to write a tutorial [1].
The main steps are as follows:

1. Use a specific title.
2. Begin with a picture of the completed project.
3. Give an overall description of the project.
4. List your materials and tools.
5. Use pictures for each stage of the process. Use a consistent background for your photographs.
6. Describe each of the steps.
7. Check for errors (if possible, ask a friend to check for errors too).

As an example of a great tutorial Bella Dia links to Future Girl's fabric lining for a crocheted bag [2].