If you don't already have a line voltage inducer, you'll need to make one (the one built into DreamPi USB modems won't work). Start by cutting open your phone line, cutting the red wire, then soldering the positive and negative of a power supply that's at least 12v 500mA to the ends of the red wire.

Once you've got your LVI, all you need to do is flash the correct image to your Pi. If you're using a Pi 4 or Pi Zero 2, download this build, otherwise download this build. After flasing the Pi, plug in the USB modem and ethernet, then power. If you're using Wi-Fi, you'll have to set that up (Username: pi, Password: raspberry) then restart the pi. If everything is set up properly, both lights on your modem should eventually come on.

Finally, plug your WebTV into the phone line and try connecting. With any luck you should properly dial in and be taken to registration.