If multiple effects modify how damage will be dealt, the player who would be dealt damage or the controller of the permanent that would be dealt damage chooses the order to apply the effects. For example, Mending Hands says, "Prevent the next 4 damage that would be dealt to target creature or player this turn" and Lava Axe is a red sorcery that says "Lava Axe deals 5 damage to target player." Suppose a Lava Axe controlled by a player who controls Fire Servant would deal 5 damage to a player who has cast Mending Hands targeting him or herself. The player who would be dealt damage can either (a) prevent 4 damage first and then let Fire Servant's effect double the remaining 1 damage, taking 2 damage, or (b) double the damage to 10 and then prevent 4 damage, taking 6 damage.