In a circular motion, run your heatsink over the 600 grit piece slowly. Since the paper is wet/dry sandable, you can put a few drops of water on the sheet directly to cut down on air borne dust. It provides a bit more lubrication also.
The heatsink should have a few swirl marks in it now. Do not worry, after we finish the sanding process, these will be gone. Continue using the same circular motion using all the pieces from 600 grit to 2000 grit. At the end, grab a cotton swab and dab some polish compound onto your heatsink.
You can use a rotary tool with a buffing attachment to do this part. Or grab a towel or clean clothe and rub it thoroughly onto the surface. Should take about 3-5 minutes. The final result should look something like:
Here's the before and after shot:
Quite a visual difference in shine and reflection. Before lapping, the temperature of our P4 2.4B @ 2.9Ghz was 40.4 C. After lapping, we managed to lower this to 38 C. Although the chip was not able to overclock higher, the lower temperature is always nice for the processor in the long run. A fried chip is not good for anyone.
The sand paper is available at pretty much all hardware stores. Go at it slowly and you'll come out with a great shine just like ours.