Unscrew heatsink.
Wipe off gunk with something like Akasa TIM Cleaner, final wipe with some meths.
Apply a small (the size of half a grain of rice) amount of the new compound to the dry core.
Put heatsink on, but not screw it on, and give it a little twist (squeezes any air from any gaps formed in the compound when putting the heatsink back on).
Screw heatsink on. Make sure you don't flex the card when tightening the screws.