If multiple effects modify how damage is dealt, the player being dealt damage, or the controller of the permanent being dealt damage, chooses the order in which to apply the effects. For example, the ability of Decorated Griffin says “{1}{W}: Prevent the next 1 combat damage that would be dealt to you this turn.” Suppose you control a Decorated Griffin, and you and an opponent are the chosen players for a Bitter Feud. If a creature that player controls would deal 3 combat damage to you, and Decorated Griffin’s ability has resolved once, you can choose to either (a) apply the effect from Decorated Griffin first and prevent 1 damage, and then let Bitter Feud’s effect double the remaining 2 damage, for a result 4 damage being dealt to you, or (b) let Bitter Feud’s effect apply first and double the damage to 6, and then apply the effect from Decorated Griffin to prevent 1 damage, for a result of 5 damage being dealt to you.