Just to be clear, you must have the ability to cast it too You must have Student level skill(6) in Magic of Miracles, have an 18 Int score, and character level 15. Other than that, just drag it to your hotbar like any other spell.

Pro tip: Don't cast Levitation when you have enemies on either side of the hazard you're crossing. Why? The spell doesn't last long and you and your companions can get caught in the hazard when it runs out. Use ranged attacks to take out nearby enemies on the other side of the hazard.