The easiest way is to start with a bike that has horizontal dropouts because, on a single speed you don't have the derailleur's slack take up arm so you need a way to adjust the chain tension.

The next issue is how to replace the multiple rear cogs:

If you have a spin-on freewheel, a single speed freewheel is a direct replacement. They come in two cog thicknesses so you need to match the chain that you are using. You also need your new rear cog to line up pretty well with your front chainring. Usually the middle chainring position works best.

If you have a cassette rear hub you'll need to buy or make some spacers to replace the cogs that you aren't going to use. The nice thing about this is that you can adjust the position of your rear cog to match the chain line of your crankset.