Well, my husband, mostly. I married him for his bike tools. And he had done a couple of conversions before we did mine.

But as with everything else, Sheldon Brown has the best information. I started by using his suggestion for threading a BMX single speed freewheel onto the existing hub, and we used the original crankset (just the smaller chainring). I recently replaced my wheels and moved to a track wheelset with a flip-flop hub, but that was because I wanted new wheels -- the BMX freewheel actually worked great and I had no complaints.

Well, one complaint: practically every freewheel we've owned has required a different tool to remove it, and that includes the same size freewheels sold under the same brand name. Stick with Shimano because your bike shop is most likely to have the right tools for those ones. Sheldon has a gear calculator that can help you figure out what size freewheel you want, but if you are guessing, lots of people ride 16/42 on singlespeeds and consider that a good around-town gear.

Most of the other work is just a matter of stripping off the stuff you don't need and replacing/overhauling anything that needs it: you might want new handlebars, and you won't need shifters, and you can take the opportunity to swap out the brakes if you don't like what you have. We had mine in rideable condition in just a few hours, although I later made some changes to the gearing, etc.