I started to write something up, and pretty quickly realized it was going to be pretty much like a manual if I covered all the bases...so, let me suggest you take a look a Partitioner and then post specific questions on using it. That will save me a lot of work, and focus on what you need to know, not what I might think you need to know.
Basically it boils down to this:
1) create a mount point for the drive
2) reformat the existing partition on the drive to ReiserFS
3) set the fstab options to allow any user to mount the drive at the mount point with write permissions
4) mount the drive as root
5) assign r/w access to the mount point for the user(s)