Well... You have to make an easiest as possible approach to the topic:
-Language: Think of your target readers, if they don't know anything about computers use a non-techie language. Also, describe lways what they have to do and what are the visual results of what they do best with...
-Screenshots: take as many screenshots as possible. It's also a good practice to add some labels or arrows to point to the items on screen refered to in the text. It's also good to crop the screenshots to the most interesting part of the screen, like where they need to fill in or where they have to press.
-Format: It's best in HTML, so anyone will be able to read it. If you don't know HTML, use some WYSIWYG editors which are pretty easy to use. You may also export it to PDF for people to print it, but that's certainly more difficult.
And the final advice: Re-read it several times, find weak explanations and strengthen them.
Good luck.
Edit: You better put it somewhere in the net and not here, because it will be sort of off-topic to the board. Get some free hosting and index it on google.
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