You're gonna have a tough time getting the result you want with the spray cans and available sand paper you have. It would be wise, and inexpensive, to invest in some wet/dry sand paper in 440/600/1400 grit. Final shine comes with buffing and a polishing glaze. A lot of work to do by hand given how fine a grit it is.
A spray gun can be obtained inexpensively. I just got an ad from Harbour Freight (probably find them online) and they had a small gun that would work for like $9. Compressor is a tad more, but you may have a friend that has one, or everything you need for that matter.
Primer first. Color sand lightly with 440 or 600 grit wet dry with lots of water between coats. Go several light coats. Resist heavy applications and possible runs (makes more work). Let dry well between coats. When you apply clear coat, repeat process with wet 600 grit, 1400 for final sand, then lightly with 2000. Use plenty of water in all sanding stages. Bugs, sand and light debris that gets in the paint (assuming you're doing this outside) will be color sanded and not show.
Make sure to wash entire bumper well and repeatedly with soap and water before starting to remove oils and grime accumulates on rear bumper. Removal is always going to give best results and make working on it far easier, particularly in hard to get to areas. Remove tail light lenses at the absolute least.
Anyone can do a crappy, half assed job, but it sounds like you want a decent result. If you can't do any one of these steps, save up a couple of hundred bucks and get it professionally done. In the end you'll be glad you did... and lose the 220 grit. Save it for another project 'cause it's too course for your purposes. Sandpaper is like $1 a sheet. Get one or two of each.
Buffing is a tricky thing. Too much, or too hard, will burn (or remove) the finish you're trying to bring out. Rubbing compound will haze the finish. Go easy and gradually on the final clear coatwith only the buffing and glazing compound. Most auto parts stores sell books on how to paint. It's not hard and the key to success is in how much effort you put into the prep work. Painting is the easy part.
Also make sure to have tape and newspaper handy to mask off area's you don't want painted. Hope this helps. Just did my ElCamino.
