Rule 702.19c

Trample over planeswalkers is a variant of trample that modifies the rules for assigning combat damage to planeswalkers. The controller of a creature with trample over planeswalkers assigns that creature’s combat damage as described in rule 702.19b, with one exception. If that creature is attacking a planeswalker, after lethal damage is assigned to all blocking creatures and damage at least equal to the loyalty of the planeswalker the creature is attacking is assigned to that planeswalker, further excess damage may be assigned as the attacking creature’s controller chooses among those blocking creatures, that planeswalker, and that planeswalker’s controller. When checking for assigned damage equal to a planeswalker’s loyalty, take into account damage from other creatures that’s being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that’s actually dealt. Example: A player controls a planeswalker with three loyalty counters that is being attacked by a 1/1 with no abilities and a 7/7 with trample over planeswalkers. The active player could assign 1 damage from the first attacker and 2 damage from the second to the planeswalker and 5 damage to the defending player from the creature with trample over planeswalkers.