Use the search as this has been done to death ..
Bob Timmons is the expert on this.. look at his answers here.
This has been automated in the v6 utility. I am used to doing it manually in v5 utility which is also fairly easy.
Apple doco is also good to read.
You cannot extend the guest network unless everything is up to date 7.6.3 firmware. And that is only with the TC as the main router in the network.
All apple routers can be configured as the main wireless or as the extend wireless.. so don't worry if a post uses express or extreme or TC.. they are for wireless, the same thing.