grover posted:

First, kill the breaker and verify the power is secured; you're going to be working with a hot lead and NEED to do this.

Open up your light switch box. Verify that there is a spare light switch, and that there are there black and red wires coming from the fan coming into the box. unscrew the switch that's currently connected, and pull it a few inches out of the box so you can work on it.. Disconnect the incoming "hot" wire, and pigtail it using a wiring nut (yellow or red are OK) and two 6" pieces of wire. Bend a loop in these two pigtails using a needle nose pliers, and screw it down. (Loop it clockwise so that it pulls in closer when you tighten the screw. Now connect the red wire to the spare light switch. Bend the wires in a Z so they'll push back easy, and reinstall the switches and covers.

Take down your fan. There will be two leads in your fan, one for the light and one for the fan. They're normally labeled which is which. Attach one to the red and one to the black. You're done!