When Liliana's first ability resolves, first the player whose turn it is chooses a card to discard, then each other player in turn order chooses a card to discard, then those cards are discarded simultaneously. No one sees what the other players are discarding before deciding which card to discard.

The player targeted by Liliana's second ability chooses which creature to sacrifice when the ability resolves. This ability doesn't target any creature.

When Liliana's third ability resolves, you put each permanent the player controls into one of the two piles. For example, you could put a creature into one pile and an Aura enchanting that creature into the other pile.

A pile can be empty. If the player chooses an empty pile, no permanents will be sacrificed.