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For polishing paper, I use various "shop rolls" of various widths, grits and types of paper - starting at 120 and going up to 400. Above 400, I'll use wet/dry sheet paper and oil or kerosene to keep the paper from loading up. If I'm polishing to a very high finish, I might go as high as 1200 and back down to 800.
The problem in polishing to higher and higher levels is that the speed with which the steel will rust slows down.
Most of the guns I polish I'll take up to 400 grit paper to get out all the 320-level lines, then I might give it a light pass with 320 paper again. Some older .22's from the 30's to 50's were originally polished to 240. Most guns will be polished to 320 to 400.
The advice of buying some clapped-out guns on which to practice is excellent. Go buy some really rough guns - Mosins, Arisaka's, etc - guns about which no one really cares (which is why they're available so cheaply). You can order "gunsmith specials" from some of the surplus firearms dealers - these will have various parts and pieces of guns without being whole guns. You'll need to gain expertise on polishing small parts like screwheads, pin ends, cylinders, slides, etc.
You can also start with a block or round of 4140 steel. If it has deep mill marks in it, so much the better. If I were instructing you, I'd cut you a 1x1x1 over 0.010" block of 4140 bar, then fly-cut it on a mill in such a way as to leave deep toolmarks. Then I'd give you a 10" 2nd cut mill file and polishing paper and set you off to hit a dimension - like "make the block 0.990 square with a 400-grit finish."
The first time you do something like this will be highly instructional. You have to maintain edges, dimensions, achieve a good finish, it teaches you how to use both file and paper.
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