How do I set up a roaming network using Airport Extreme (gen 5) and Express (gen 2)?

The options mentioned in this How To article http://support.apple.com/kb/HT4260 have changed. I have 6 brand new AirPorts (2 Extreme, 4 Express) all connected via ethernet cables to my network, located in various parts of our school building. I am not sure how to correctly configure each base station to create one roaming network. I've tried setting each device to "Create a wireless network" and using the same name & key. I've also tried setting one base to "create" and the others to "extend" but this doesn't seem to work. Any ideas?