Import the export file from another service. Anything already on your shelf is skipped.
Letterboxd: Settings → Import & Export → Export your data. Unzip it and pick diary.csv (each viewing, with dates) or watched.csv. Pick watchlist.csv to add films you want to see.
Goodreads: My Books → Import and export → Export Library. Pick the goodreads_library_export.csv it gives you.
IMDb: Your Ratings or Your Watchlist → the ⋯ menu → Export. IMDb emails or lists the CSV under Exports when it's ready. Pick either file; titles on the watchlist come in as "Want to watch".
MyAnimeList: Profile → Export, choose your anime or manga list, and pick the .xml.gz file it gives you (or the .xml inside it).
Other CSV: a header row with title and any of type (film, show, game, book, album, comic), creator, year, status, rating (0–5), finished (YYYY-MM-DD), notes, tags.
Your shelf lives in this browser on this device only. Keep a backup.
Erase every item on this device. Export a backup first if you might want it back.