Second, read other's examples (eg MSDN, developerWorks, Docs.Splunk)
Third, practice
Fourth, get feedback (and incorporate all the constructive bits)
Fifth, practice
Sixth, focus on why more than how; leave out used to be in favor of this is now; be concise; crosslink other docs
Seventh, make it maintainable - you do not want to "own" this forever and always...eventually you won't be there: make it something someone else can pick up and run with (and not have to recreate from scratch)
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Some resources I've written (and a Wikipedia page):
- https://writing.stackexchange.com/a/33505/1317
- https://writing.stackexchange.com/a/33578/1317
- https://pm.stackexchange.com/q/799/167
- https://en.wikipedia.org/wiki/DIKW_pyramid
- https://antipaucity.com/2013/10/11/theres-never-enough-docum...
- https://antipaucity.com/2013/02/10/organizational-knowledge-...