When using convoke to cast a spell with {X} in its cost, first choose the value for X. That choice, plus any cost increases or decreases, will determine the spell's total cost. Then you can tap creatures you control to help pay that cost. For example, if you cast Wrath of the Skies (a spell with mana cost {X}{W}{W}) from exile, choosing 3 as the value of X, the total cost is {3}{W}{W}. If you tap a white creature and a red creature, you'll have to pay {2}{W}.