On mouse/KB, after selecting the cannon you pick the cannonfire action on the new bar that pops up. Hold the right mouse button and move the camera to aim the circle on the ground (it defaults to the center of your screen). It will be orange if you are aiming somewhere in range, purple if not. After that you click the left mouse button to fire on that spot and anything within the circle will take damage.

If you have used the caster limit break before it works the same way.