Are you adding a new switch so you can control both of them separately?
The hot lead would get attached to the white from the 12/3, that white would get jumpered to one terminal of each switch (assuming you are going to use a separate switch for both the fan and light). Then use the black on the other terminal of one switch and the red on the terminal of the other. Back at the fan/light, connect the black to the one where you want the switch with the black on it to control, and the red to the other. You should put some black tape on the white lead at both ends to identify it as being hot.
Basically, you need to jumper the hot wire to the switch(es), and then through the switch(es) back to the things you want to control.
i have a light fan combo that i'm using to replace a standard light in my bathroom since i have a major moisture problem and the house was built in the 50s. i have my black and white (hot and common) wires coming from the breaker and going directly to the light. I have new 12/3 wire running to the switch from where the light is. i need to know where to send my power from the breaker thru the switch and make it function properly.
