There's nothing to start: just open the same note. When two or more people have a note in a shared vault open, it becomes live:
- you see each other's avatars at the top and cursors in the text, with names,
- edits appear as they're typed,
- edits merge automatically, so nobody overwrites anyone.
Close the note and it goes back to syncing normally. The live session leaves versions in the history like any other edit.