First, the old Extreme needs to be reset back to factory default settings.....if you have not already done that.
Power off the old Extreme for a minute
Hold in the reset button on the back first, and keep holding it in for an additional 10 seconds while you simultaneously plug the power cable back into the old Extreme
Release the reset button after the hold period and allow a full minute for the old Extreme to restart to a slow, blinking amber light status.
Plug the Ethernet cable into the WAN "O" port on the old Extreme.
Click the AirPort icon at the top of the Mac's screen and look for a setting of New AirPort Base Station
Click on AirPort Extreme just below the setting

The setup utility will open up automatically and suggest the proper setup to extend the new AirPort Extreme network
Enter in a name that you want to call the old Extreme and click Next.
Things will look similar to this:

Now watch the screen to confirm that the old Extreme is being set up to extend using Ethernet.....like this:

The utility will set up everything for you automatically. When you see the message of Setup Complete, click Done
That's it.