This recipe will keep your trakt watchlist in sync with your imdb watchlist. It is a one way sync, so direct changes to your trakt list will be overwritten by the imdb watchlist. This example is adaptable to sync any imdb list to any trakt list.
tasks:
  imdb_to_trakt:  # This task adds all the movies in your imdb watchlist to your trakt watchlist
    imdb_list:
      user_id: ur26726824
      list: watchlist
    accept_all: yes
    trakt_add:
      username: tom3297
      password: Atermial1!
      list: watchlist
    disable_builtins: [seen](/seen)  # If you wish to be able to remove things from the trakt list directly, without them being re-added, remove this line
  imdb_to_trakt_remove:  # This task removes anything in your trakt watchlist which is no longer in your imdb watchlist
    trakt_list:
      username: tom3297
      password: Atermial1!
      list: watchlist
      type: movies
    crossmatch:
      from:
        - imdb_list:
            user_id: ur26726824
            list: watchlist
      fields: [imdb_id](/imdb_id)
      action: reject
    accept_all: yes
    trakt_remove:
      username: tom3297
      password: Atermial1!
      list: watchlist
    disable_builtins: [seen](/seen)