601.2b

If the spell is modal, the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the spell (see rule 702.47), they reveal those cards in their hand. If the spell has alternative or additional costs that will be paid as it’s being cast such as buyback or kicker costs (see rules 118.8 and 118.9), the player announces their intentions to pay any or all of those costs (see rule 601.2f). A player can’t apply two alternative methods of casting or two alternative costs to a single spell. If the spell has a variable cost that will be paid as it’s being cast (such as an {X} in its mana cost; see rule 107.3), the player announces the value of that variable. If the value of that variable is defined in the text of the spell by a choice that player would make later in the announcement or resolution of the spell, that player makes that choice at this time instead of that later time. If a cost that will be paid as the spell is being cast includes hybrid mana symbols, the player announces the nonhybrid equivalent cost they intend to pay. If a cost that will be paid as the spell is being cast includes Phyrexian mana symbols, the player announces whether they intend to pay 2 life or a corresponding colored mana cost for each of those symbols. Previously made choices (such as choosing to cast a spell with flashback from a graveyard or choosing to cast a creature with morph face down) may restrict the player’s options when making these choices.