502.46a.

Convoke is a static ability that functions while the spell is on the stack. "Convoke" means "As an additional cost to play this spell, you may tap any number of untapped creatures you control. Each creature tapped this way reduces the cost to play this spell by {1} or by one mana of any of that creature's colors." Using the convoke ability follows the rules for paying additional costs in rules 409.1b and 4091f-h.

Example: You play Guardian of Vitu-Ghazi, a spell with convoke that costs {6}{G}{W}. You announce that you're going to tap a colorless creature, a red creature, and a green-and-white creature to help pay for it. The colorless creature and the red creature each reduce the spell's cost by {1}. You choose whether the green-white creature reduces the spell's cost by {1}, {G}, or {W}. Then the creatures become tapped as you pay Guardian of Vitu-Ghazi's cost.