It depends on who is going to read.
You must speak the language of the readers, write according to the level of knowledge of the readers.
Imagine a Person who is the Reader, what how knows ? what he must learn ? e.t.c.
The language must be simple, and accurate, to use images and diagrams.
For the structure see othr similar manuals.
Someone else must read the manual and suggest corrections.
google says: How do you create a user document?
Tips For Writing Better User Documentation
- Start With A Plan. ...
- Write in Plain Language. ...
- Use Visuals to Speed Up Understanding. ...
- Break Complex Tasks Into Simple Steps. ...
- Follow a Hierarchy That Makes Sense. ...
- Make It Searchable. ...
- Include a Table of Contents. ...
- Test, Analyse & Reiterate
