As the combat damage step begins, the active team announces how each attacking creature will assign its combat damage. Then the defending team announces how each blocking creature will assign its combat damage. See rule 510.1.
May 3, 2019 · War of the Spark
Once blockers have been declared, for each attacking creature that’s become blocked by multiple creatures, the active team announces the damage assignment order among the blocking creatures. Then, for each creature that’s blocking multiple creatures, the defending team announces the damage assignment order among the attacking creatures.
January 25, 2019 · Ravnica Allegiance
Once blockers have been declared, for each attacking creature that's become blocked by multiple creatures, the active team announces the damage assignment order among the blocking creatures. Then, for each creature that's blocking multiple creatures, the defending team announces the damage assignment order among the attacking creatures.
June 5, 2017 · Commander Anthology
Once blockers have been declared, for each attacking creature that’s become blocked by multiple creatures, the active team announces the damage assignment order among the blocking creatures. Then, for each creature that’s blocking multiple creatures, the defending team announces the damage assignment order among the attacking creatures.