you're going to need some more spacers. but you remove the cassette, which is made up of 9 cogs and a bunch of spacers. Then you put some spacers on, throw a cog on, add some more spacers, and top it off with a lockring. Put the cog in line with your chainring and you're golden.
Should be about 15, 20 dollars from just about any shop
How redneck are you? Because I used PVC pipe from Home Depot for the spacers. A 10' stick costs just under $4.00, and that gives you just enough extra to mess up a couple hundred times cutting the pieces to size. I bought a BMX cog, but you could probably find a way to break up the cassette if you were so inclined. Then just put the lockring from said cassette on top.
I thought maybe someone in Home Depot would be curious as to why I had a rear wheel in the store, but true to Home Depot employee custom, none could be found the entire time I was there. I swear, the first thing they teach their employees on day 1 of orientation is to say, "I don't know. This isn't my department."