Assuming that you are planning to extend using a wireless connection, this Apple Support document covers the process:
The example shows an AirPort Extreme "extending" the network, but an Express would be configured exactly the same.
If you are not using the 10.5.8 operating system as your profile states, and have moved to Lion or Mountain Lion, the setup process is different. Post back for more info if that is the case.
It is pretty simple. Use AirPort Utility and follow the instructions here:
Notice it suggests the following:
If you have configured your Wi-Fi base stations in the past, it may be helpful to do a factory default reset of each Wi-Fi base station that will be part of the wireless extended network before you begin.
I am using Mountain Lion (10.8.2) and i set up the airport express as an extended network to my Time Capsule base station, but the express will not show up on the wifi of any of my devices (macbook pros, iphones, ipads, etc.). airport utility shows a straight line connecting Internet to Time Capsule, but a dotted line connecting Time Capsule to airport express (not sure this is relevant). also, all the lights are green and the internet works. oh, and airplay to the express works.
Internet--------------Time Capsule- - - - - Airport Extreme
i set up the airport express as an extended network to my Time Capsule base station, but the express will not show up on the wifi of any of my devices
The Express is extending the network. It will not show up as a separate wireless network. It will appear as if you have one wireless network to all devices. This is normal.
Enjoy your network.