The KDE4 human interface guidelines are meant to be concise and easy to read. Favor the use of bulleted lists when writing guidelines for an item. Use images to show what's being described as much as possible, and link to other items within the guidelines when needed.
The most up-to-date guidelines can be found here - however don't consider them final yet. They might still need review.
The usability wiki is a list of items brainstormed for the KDE HIG - but most of them are out of date. Use those only as a starting point for finding what guidelines still need to be written.
Use the current KDE guidelines and the HIG as a resource while writing. A general outline to get started:
Also, if necessary: