I don't want to use it as a wireless network. I just want it to back up my computers. How do I set it up?

Plug it into the main modem which must also be a router.

Put the TC in bridge mode..

Then go to the wireless page and either turn it off.. but I recommend roaming.. as the TC is much faster on the 5ghz band and backups can drag down wireless performance of the whole network.

Roaming setup thusly. ...just give the TC the same wireless name= SSID as the main router.. same security.. WPA2 AES= WPA2 Personal and the same security password.. and then the computer will simply pick the best wireless to connect to.. without creating a second wireless network.. although it is not really such an issue to do so.