If multiple effects modify how damage will be dealt, the player who would be dealt damage or the controller of the creature 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 any target this turn." Suppose a spell would deal 5 damage to a player who has cast Mending Hands targeting themselves. The player who would be dealt damage can either (a) prevent 4 damage first and then let Stronghold Furnace'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.