Let's review what La Pastenague has advised.
1) Reset the extra AirPort Extreme that will be added to your network back to its default settings. To do this.....power up the AirPort Extreme and let it run for a minute. Then, hold in the reset button on the back of the AirPort Extreme for 7-8 seconds and release. Allow a full minute for the AirPort Extreme to restart to a slow, blinking amber light status.
2) Plug it from its WAN port to the main airport LAN port. So, you would connect an Ethernet cable from the WAN "O" port on the extra AirPort Extreme to one of the three LAN <--> ports on your main AirPort Extreme. Choose any of the three ports that you want.
3) Just run through the setup in the airport utility. To do this, you would.......
Open Airport Utility
Click on the Other Wi-Fi Devices option
Click directly on AirPort Extreme
When you click on AirPort Extreme, that will start up the AirPort Utility setup "wizard". The wizard will take a few seconds to analyze the network and then display the correct setup with a screen that will look similar to the example below, except that you will see AirPort Extremes displayed and the name of your wireless network
Type in a device name that you want to call the extra AirPort Extreme. Example.......AirPort2
Click Next
Watch the next screen very carefully to see that the AirPort Extreme is being configured to extend using Ethernet. This will confirm that the setup wizard is applying the correct settings to AirPort2.
When you see the message of Setup Complete, click Done.......and you are done.