I once polished a GM TPI intake. Man that was a lot of work!!
I started with a wire brush on a drill and cleaned the crap out of it. They used to make a sponge type thingy that was like emery cloth. Thats what I used. I think 3M sold it, but it's been years since I did that.
2 or more ways to look at it really, if its a 'cast' aluminum like our IM's you will have alot of bumps and ridges to contend with and knock down to get a proper polished / mirror look. You should start with a low grit aluminum oxide paper (ie. sand paper) in the range of 120-220 grit. your better off and the sanding will go alot faster if your wet sanding, so get in the TUB but be prepared for extra clean up as Aluminum dust will lightly stain ceramics. If you use a plastic bin with water in it that would be better.
Use a sanding block on all the flat surfaces so that they stay FLAT, don't finger sand those areas.
So you got it flat and relatively smooth with the 120-220, keep moving up the scale to reduce those 120-220 scratches.
Go to a 320-400 paper and wet sand A LOT MORE.
Go to a 600-800 paper and wet sand A LOT MORE.
Go to a 1000 paper, this is where you should take a close look at the remaining scratches, if it looks like your not removing them with the 1000 paper go back down a stage to remove them then go back up.
Now you could go onto 2000 and then for an absolutely show quality shine/polish go to 3000-4000 polishing pads but those are very professional and expensive so why bother.
Get some metal mag/Aluminum polish (Meguire's, Caswell make one) and polish it up to that high gloss. Then you have to look at keeping it polished, ZOOP Seal is the best or you can just clean and polish it every so often
Now if its a sheet material you can just polish it with the polishing compounds pretty easily. But no step in metal polishing is ever easy work. Once you start you need to stay comitted.