MTG Keyword Rules — Comprehensive Rules Glossary

Harmonize represents three static abilities: one that functions while the card is in a player’s graveyard and two that function while the spell with harmonize is on the stack. “Harmonize [cost]” means “You may cast this card from your graveyard by paying [cost] and tapping up to one untapped creature you control rather than paying this spell’s mana cost,” “If you cast this spell using its harmonize ability, its total cost is reduced by an amount of generic mana equal to the tapped creature’s power,” and “If the harmonize cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.” Casting a spell using its harmonize ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.

You choose which creature to tap as you choose to pay a spell’s harmonize cost (see rule 601.2b), and then tap that creature as you pay the total cost.