As Smallpox resolves, first each player loses 1 life at the same time. Next, each player chooses a card in hand without revealing it, then all of the cards are discarded at the same time. Then starting with the player whose turn it is and proceeding in turn order, each player chooses a creature, knowing the choices made before them. All of the creatures are sacrificed at the same time, then finally repeat this process for lands.

Any abilities that trigger while Smallpox is resolving will wait to be put onto the stack until Smallpox is done resolving.

Any abilities of the lands sacrificed may modify or trigger on the creatures being sacrificed and on the cards being discarded. Abilities of the creatures won't modify or trigger on the lands being sacrificed, but may modify or trigger on the cards being discarded.