I'll give you the rundown.
Materials:
600, 800, 1000, 1500, 2000 grit wet/dry sandpaper
A mirror or piece of glass at least 12" x 12"
Electrical tape
Water
Brasso
A couple pieces of tissue
Compressed Air (for getting things dry when you are done)
Towel (See compressed air)
*With the sandpaper; If your block is already pretty flat and smooth, you probablly can start with the 800 or 1000 grit.
1. Find a flat surface and lay the mirror down upon it.
2. Tape down the sanpaper to the mirror with the electrical tape (Try to get as good of a seal as possible)
3. Wet down sanpaper (this will aid in carrying the copper particles away plus aid in friction)
4. Add a small drop of liquid soap to the base of the heatsink
5. Place heatsink on sandpaper and stroke or lap back and forth in a straight 5" line or so appox. ten times in each direction.
6. Turn heatsink 90 degrees clockwise and repeat the lapping. Turn 90 degrees again, lap.
7. Check bottom of heatsink to see if it's now flat with even scratching on the bottom. If not, continue with steps 5 and 6 untill it is.
*Remember to periodically clean the copper particles off, and add water.
8. Once heatsink is flat with uniform scratches, move up to a higher grit paper. Follow steps 5 and 6 again. You shouldn't need to lap as much after the first lower grit, since all you doing from here on out is sanding away the larger scratches left by the previous grit sandpaper.
9.Continue these steps up till after you are done with the 2000 grit. By this time your HSF should be very flat and smooth. Now to make it look like a mirror, on to step 10.
10. Apply a small amount of Brasso to a piece of tissue. Rub it into the bottom of the heatsink with a small fast circular polishing motion.
DO NOT USE ON ALUMINUM
11. Wipe clean, and use some Arctic Clean or alcohol to clean.
12. "You look marvelous!"

You now have a beautiful copper mirror on the bottom of your heatsink.