Identify your audience before you begin writing. How much knowledge do they already have about this topic? Will they need every term explained to them or are they already experienced with the topic?
If you are writing for two very different audiences, such as new users and experts, consider writing two separate manuals or including separate sections aimed at each group. For example, you can have introductory chapters for the new users that the experts can skip over.
Design a writing plan before you begin the manual. You may wish to start with a summary that explains all sections of the manual, followed by a product description and materials required for the task. You can then move into operating instructions, with a final section on troubleshooting.
Perform a task analysis of all the things you need to teach your audience, and then group similar tasks together. For example, when explaining new software, group together all tasks concerning installing and starting the software.
Don’t be afraid of addressing the reader directly as “you”. Manuals are meant to be informal and the instructions need to be clear for the reader. Offer instructions in the imperative, starting with a verb. For example, “Open the document named ABC.” or “Attach part A to part B.”
Number or list instructions whenever you can. Formal paragraphs are unnecessary when writing instructions and can impede progress. Numbers and bullet are much easier for the reader to follow.
Be specific with all instructions. Don’t write, “Use a small piece of wire”. Write instead, “Use 10 cm of wire”.