Find a light sanding block, I use a cheap plastic one from the hardware store. This keeps the sandpaper flat.
Start with 600 grit.
Then 800 grit.
Then 1000 grit.
Then 1500 grit.
Then 2000 grit.
..you can go higher if you want, but it really isn't necessary.
...you can POLISH for a mirror finish if you want to, but it isn't necessary and WILL NOT affect your temps.
You SHOULD sand in one direction, up and down, for the most even lap job. Choose the direction that the factory finish was done in (you can tell from faint cut lines on the block).
Once you get up to the 1000 grit, you may use a soap (I use dish soap) to help with the contact between the sandpaper and block.