I agree with @SnOrfus: look around for a good example and copy the template.
You know already that almost all end-user documentation just sucks; it's so, so bad: wordy, lots of extraneous BS, full of weasel words -- "it is recommended that," "the user is urged to," all crap like that. Just 90% pure hooey. And, by the way, remember that all of these so-called "user guides" are around 1200 pages long and cost $89.95.
There surely are a few good examples around, though. Kernighan and Ritchie, C Programming Language is the best example of a good end-user doc that I know of. It's short; it uses normal, everyday words; it's complete; it's correct. A beautiful work of art, in my opinion, that fits comfortably in 256 pages (first edition).
A good forum for this question is the writers' group on StackExchange. You'll find discussions of end-user technical docs over there sometimes.