Rule 802.4 · Attack Multiple Players Option
If more than one player is being attacked, controls a planeswalker that’s being attacked, or protects a battle that’s being attacked, each defending player in APNAP order declares blockers as the declare blockers step begins. (See rule 101.4 and rule 509, “Declare Blockers Step.”) The first defending player declares all their blocks, then the second defending player, and so on.
Subrules
A defending player can block only with creatures they control. Those creatures can block only creatures attacking that player, a planeswalker that player controls, or a battle that player protects.
When determining whether a defending player’s blocks are legal, ignore any creatures attacking other players and any blocking creatures controlled by other players.