The text should be as clear as possible. Keep the vocabulary simple and straightforward, with definitions where they might be helpful. Do not introduce initialisms or acronyms that are not common or necessary. Use the same terminology to refer to the same concepts, items or actions throughout the document, to help your reader to keep things straight.
Be direct and tell the user exactly what to do. With instructions, the imperative voice is appropriate and useful:
Lift the red lever on the left to release the catch, then open the door.
not
The door may be released by means of the red lever situated to the left.
The text should also be self-contained; it should include all the information that the reader needs to use a product or follow a process – but no more. Instructional texts are not the place for a lot of background information.
See Clear and appropriate language for more information on writing clearly.
