If it's the 327W Westell router/modem ... tell them to go to
http://192.168.1.1 and login... then hit Configuration --> Wireless --> Basic. You will see options for the following:
Wireless Operation (enable this)
Network Name (SSID) (set this, here and same name for the wireless machine)
Channel (whatever)
Mode (whatever)
4x Support
Hide SSID (enable this)
Then go to Wireless --> Security:
(use WEP or WPA - I use the latter).. it's pretty self-explanatory... then you do the same thing for your wireless card... set it to the same key.
Then go to Wireless --> Mac Filtering:
(enable mac filtering - and enter the mac address of your wireless card on this screen. To find out the MAC address of the wireless card, type "ipconfig /all" and you'll see it.)
And that would be all-she-wrote...
With mac filtering, hiding SSID from broadcast, and WEP/WPA encryption, you should be as secure as you can typically get with wireless.