The 30 second overview:

1) On title screen, click "Metaverse"

2) Sign in, create a character if necessary.

3) Start a game. Just like a normal sandbox game, but you can't use mods.

4) At the end of the game, you will be given the option to post the score to the Metaverse. If you didn't play the game entirely in one session, you will need to sign in again to post the score.

That's the quick version. I think there's a more elaborate version in one of the sticky posts.