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You're lucky to have time to learn online help and plan it well. Jessica
Lange gave great advice--read AND follow what the experts say. I'd suggest one
more thing: Find a well-written online help file(s) and analyze what the author
did.
I was lucky enough to start working on a well-done help system as it was being
prepared for its third release. Despite reading the books and having a fine
example, I still made mistakes. But, the excellent example helped me find those
mistakes. (I also have to give credit to the other writer, Kelly, who showed me
what I needed to achieve and helped me see where I'd missed the mark.)
I constantly check my work against the design and the style guide. On new help
systems, I take the time to design. Help systems ARE software systems, they need
to be designed as consciously as the software products they document.