flapwheel, sandpaper going from grit 60 all the way to 2000 in steps, and A LOT of patience and elbow grease.
basicly, you use the flapwheel to smooth out all the original casting roughness.. and start with 60 grit sanding everything horizontally. When this is done, move one step finer (80 or 100) and start sanding vertically. This way you can see if you sanded everything and there's no sandingmarks from the previous grit left. Using this method, work your way to a finer grit everytime, switching direction and make sure you sand out EVERY mark left by the previous grit. If you don't, you'll see it in the endresult.
after you're done with 1500 or 2000 grit and you're satisfied, use some polishingcompound a couple times (I believe a brand called 'mothers' is very popular in the US) and keep going until satisfied.
Be prepared to invest a lot of time and remain patient, and also lose your fingerprints for a few weeks. But take your time with every step, that's the most important. Trust me, rushing it will spoil it. I've done 4 valvecover and 2 intake mani's for my Probe GT by hand.