The script automatically checks for kudos you left. To mark a work as seen, use the "Seen ✓"/"Unseen ✗" buttons. (Disclaimer: "Unseen ✗" button doesn't make your brain unsee the thing.) The "Seen works" dropdown menu appears wherever there's a list of works or bookmarks.
Kudosed works are treated like seen, except they get the bonus kudos image. So if you leave kudos, there's no need to mark the work as seen.
Remembers when you bookmark a work. Page through your bookmarks list once to make it remember the works you bookmarked previously (shhh just do it).
If you're logged out the first time you visit AO3 after installing the script, it will ask for your username.
Much ado about buttons:
Seen ✓ — click to mark the work as seen
Unseen ✗ — click to remove the work from your seen list
Import/export your seen list — export your current seen list for backup or import a saved one
For all works on this page:
Mark as seen — mark all works on the page as seen (tip: your bookmarks or history)
Unmark as seen — remove all works on the page from your seen list
Re-check for kudos — re-check all works on the page for kudos (tip: your history after you kudosed things in a different browser/on mobile)
Check for bookmarks — check all works on the page for bookmarks you left
Hide seen works: NO/YES/COLLAPSE — set whether to hide or collapse (show only the header of) blurbs of works you kudosed or seen on lists
Highlight bookmarked: NO/YES — set whether to highlight bookmarked works on lists
Mark as seen on open: NO/YES — set whether to automatically mark every opened work as seen