As Liliana’s Triumph resolves, the opponent whose turn it is (or, if it’s your turn, the next opponent in turn order) chooses a creature to sacrifice, then each other opponent in turn order does the same, knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time. Then, if you control a Liliana planeswalker, in the same order, each opponent chooses a card in their hand without revealing it, and those cards are discarded at the same time.