First you'll need to download Flycast and the WebTV Redialed Dreamcast disc image. You can get Flycast here, and the disc image here.

Once you have both of these, extract the disc image and open Flycast. Load the disc image, and you should boot into WebTV.

If you'd like, you can switch to keyboard controls by hitting "Tab", then selecting "Settings", "Controls", and changing Device A from "Sega Controller" to "Keyboard".

Once your controls are properly set up, hit "Start" ("Home" if you're using keyboard controls) to progress past the title screen, navigate to "Connect to WebTV", and select "Register". With any luck you should be taken to registration.