I've just added a paragraph to the RPi Beginner's wiki page that points to another wiki page that describes how to attach, mount, and unmount USB flash or hard disk drives to your Pi:
http://elinux.org/Adding_USB_Drives_to_a_Raspberry_Pi
Some beginners should try out the instructions there and let me know how they might be improved if there is anything that isn't crystal clear. I need to add or link to another page on creating/modifying drive partitions, as noted by the [TBD] in the wiki page.