First, you will of course need to have a Comcast account with a username and password. Then, go to this URL. You'll see a page that looks like this.
After you log in, the URL above will take you directly to the “My Current Data Usage Page.” My (surprisingly modest) page looks like this:
It's that easy.
Note the comment in the paragraph at the top: “…enforcement of the 250 GB data consumption threshold is currently suspended.” There's more detail on the why of that in: “How Comcast is Now Handling Internet Data Usage and Caps.“