Connect the Ethernet cable to the WAN "O" port on the Express and power it up. Wait for the Express light to blink amber slowly
Open Macintosh HD > Applications > Utilities > AirPort Utility
Look for the tab in the upper left hand corner of the AirPort Utility window that reads Other AirPort Base Stations (1)
Click on this tab, then click on AirPort Express xxxxxx
AirPort Utility will take a minute or two to analyze the devices and settings on your network, then it will announce that the AirPort Express will setup to extend the wireless network
Edit the name of the AirPort Express device to read as you wish, then click Next
Wait another minute or two while AirPort Utility configures the AirPort Express
When you see the message that Setup is complete, click Done and close AirPort Utility
You are all set
via Hardwire Airport Extreme to an Express: Apple Support Communities.
Tags: airport express, airport extreme, apple router, networking