Follow the instructions in the spell's text.

If the enemy has failed his dispel attempt (or not even attempted one!), the spell is cast successfully and its effect is now resolved. Each spell in Warhammer provides all the information you need. Many spells inflict hits or wounds on your enemies – you can find out how to resolve these in the Shooting phase chapter.

Remember that if your spell was cast with irresistible force, you'll need to go back and resolve the result of the miscast as soon as you've finished resolving your spell effect.