Well you start for almost 400 grit, to get the main rough bits out, you rotate the heatsink atleast 2 times while grinding it. Do not put force on it, just use the weight of the heatsink. Then slowly work your way up to 1600 or 1800 grit paper. You can get a good pack of sandpaper at any good supermarket. Look don't go completely by my advice, just go online and find a guide, because I am just giving you a shortened version of the thing.
Also i will recommend a beer or a cola, which ever you like, it can be really useful. (to drink of course)
Walking in the dark