These ideas you have already presented form the basis of the manual you will write. For example, manuals ordinarily start by addressing their readers directly, specifying who will benefit from reading the manual and why. And the introduction overviews the manual's organization. So you may find that the easiest place to start writing the manual is with the introduction, which will remind you what it's all about. Later you may have to come back and tinker with this section, because you may or may not include what you originally thought you would.
Clarity is the hallmark of the language of manuals, which are task-oriented.
No matter what your topic, if you are writing a manual, then you want your reader to be able to finish the manual and go do the given activity or refer to the manual and get help from it during the performance of the activity. Keep paragraphs short. Write directly to the reader, using you and commands. Pay attention to your word choice given the tone you want the manual to have. However, if the situation is appropriate, then humor and informality can enliven and clarify an otherwise difficult and somewhat boring subject.