Turn off the electricity to the room at the circuit box. From your ladder, remove the old ceiling fixture by taking out the screws that were holding it in. Disconnect the wires of the old fixture, so they hang out of the box. Discard the fixture.
Hold up the cathedral ceiling bracket underneath the electrical box, lining up the screw holes. Attach the bracket using the provided mounting screws.
Assemble the fan on the floor according to the instructions that it came with. The kit should include a "down-rod," which is a hollow metal tube at the top of the fan, with a mounting canopy at the top of the tube and wires coming out of it.
Put the assembled fan on the top of the ladder, with the mounting canopy raised to be near the ceiling bracket and electrical box. Connect each wire from the fan to the same colored wire from the ceiling, using plastic wire connectors and wrapping them in electrical tape.
Connect the copper grounding wire from the electrical box to the green grounding screw inside the fan canopy.
Hold the canopy against the ceiling bracket, tucking in the wiring as you do. Secure the canopy to the bracket using the provided mounting screws.
Turn the electricity back on.