First write a specification for your project. Then when you are done coding it should be easy to convert the spec into a basic manual. You should not have to refer to the code at all to find out how something works when writing the manual. Note that you can save yourself some time by writing the spec in the right style (3rd person, etc.) that you want for your manual to begin with.

When writing a manual I always ask myself constantly, "if I was reading this for the first time and knew nothing, what questions would I be thinking?". And then I would write in the information that would answer the questions.

Think back to when reading other people's manuals and consider what kinds of information you found were missing and what would have made your life easier if it was in there. Then make sure you add that in to your manual.

Of course: design your product/software so the user has little need to refer to the manual in the first place. I think an intuative product is a good product. Most people don't read the manual anyway, and in that case your well written manually will really help the tech support people give accurate and complete answers.

Andy