Take a top-down approach. Produce help systems, manuals and training that complement each other. Look at core documentation first. For example, the draft design and text for a help system might form the basis for a user guide and for user training. Create manuals that can handle version updates or be changed in response to user feedback.
Many people refuse to use manuals.
The software giants test their documentation. The resulting page layouts are remarkably similar. There's consensus about what works. The writing style is lively, instructional, active. The emphasis is on use, not features.