When a character sleeps, they may elect to spend 1 point of Luck to have the DM cast their Fortune. This gets them one forthcoming answer to a question of theirs, nestled within a weird dream. This also draws 1 card from the Fortune Deck. Before the card is drawn, the player determines whether it will be played on their PC or the Table. Cards played against the PC affects only that PC directly, where playing to the Table puts the element from the card into larger play.
These represent new elements the DM has to/gets to bring into play, working it into the existing narrative.
There are other game circumstances in which the party’s Fortune might be cast, and the Fortune Deck is something many soothsayers, spellcasters, and gamblers actually have in this world, so you can usually find other ways to use it.