Pretty simple actually. Get some 400grit, 800grit and 1000grit sandpaper. You will use 4-5 sheets of each. Some like to continue with 1500grit and 2000grit - you can if you like - but it shouldn't make any difference in temps.
Get a nice flat surface, if you have some surfaced aluminum or you can use a good counter-top or marble if you have it. Basically looking for as flat of a surface as you can get. Attach your sandpaper sheet to the flat surface (tape?) and with light pressure (and keeping the heatsink flat - don't tip it!) work in slow circles. When the surface of the heatsink looks uniform, move to the next grit (400 -> 800 -> 1000 and so on). DO NOT USE POLISHING COMPOUND - it will affect the performance of the heatsink.
All there is to it! Post some pictures when you get it done!
EDIT: Forgot to mention, a lot of people like the wet-sand too.