The following assumes that you plan on using a (wifi) router connected to the modem, even though operating a computer connected directly to the modem should work the same way. It also assumes that you don't have a static IP setup with Sonic.net, in which case you'd have to enter the IP address, DNS, Gateway etc. manually into your (wifi) router or computer after completing the below steps.
1.) Connect your modem to the phone/DSL line. Attach your computer directly to the modem.
2.) Press the reset button on the back of the modem until the modem resets then releases the reset button.
3.) Once the modem reboot is complete, your computer should receive an IP address from the modem (in the 192.168.1.x range)
4) Open a browser, go to 192.168.1.1 and enter user and password. In case you see the modem's software update page, turn off the modem, wait 10 seconds and turn the modem back on. Then repeat this step. (BTW: I had difficulty performing the steps using Firefox on windows, but it worked using Internet Explorer.)
5) Use the modem's quick setup. When prompted, make sure the following values are being used:
VPI: 0, VCI:35, Service category: UBR without PCR, [NEXT], Connection type: bridging, Encapsulation: LLC/... [NEXT] Enable bridge service: <checked>, Service Name: <whatever you like or leave as is>, [NEXT], [SAVE] -> The modem should reboot at this point
6) Once the modem has finished rebooting, go to 'Advanced Setup' -> 'LAN', uncheck 'Enable DHCP server', then click 'Save & Reboot'.
7) Disconnect your computer from the modem and connect your (wifi) router to the modem.