You want to put it on the sliding surfaces of the pads to help them release properly. Also put some on the guide pins of the calipers. The sliding surfaces are the sections that touche the hardware of the bracket for the caliper. If you set the pad back in its place, you'll see where it goes. Just make sure its lube and not an anti-squealent, because the anti-squealent would go on the the back of the pad to help absorb vibration. All you need is a light coat on the sliding surfaces.
One thing you might want to do if your reusing your rotors, is sand them lightly in a non directional way so you don't glaze your new pads and rotors.