To lap:
Go to like an auto store or hardware store and get some 1000 grit, and some 2000 grit sandpaper.
Find the smoothest surface you can possibly find. A piece of glass is your best bet.
Wet the 1000 grit sandpaper, and put even pressure on the heatsinc, and rub in circles over the sandpaper. Don't really apply too much pressure, the weight of your hand and the weight of the heatsinc should be fine. Don't go in one path the whole time, go in circles one way, then the other, just rub it all over.
Then, get the 2000 grit paper wet, and do the same. If done right, you should have a near-mirror shine from the bottom.