When I polished mine, I start out with real low grit sand paper to get the dimples out. I think I was using something like 80-120 grit. Then you work your way up to 2000 or 2500, and finishing it off with an alluminum polishing compound.
Heres a picture of how mine turned out:
This is a really old picture when it was freshly polished, please dont ask about my engine covers....
Anyways, i would suggest having power tools to do the job, else...this is going to take you an extreme amount of time and elbow grease.
Also, having a spare IM to work on is great so you dont have to spend the time taking off/putting back on the manifold everytime to work on it.