I took a mixed media class that Faber Castell sponsored at the Scrapbook Expo over the summer.
They showed a few different ways to use them, but they're just not my thing, I don't think.
Anyway--one way they showed was to simply rub them directly on the stamp and stamp down. Another way was to scrape some onto a craft sheet or plate or bowl and add a few drops of water to mix into it and create a thick paint and use it that way.
There were another few methods they showed, but I don't remember them right now.