Easiest, cheapest way is to choose what gear you want, take the rest of them off, and add spacers. Store-bought they're about 10 dollars. You could get some spacers from an old cassette, or make your own out of the right gauge PVC if you have the proper tools.

Then take off the shift lever and cable, and set the adjuster screws on the der to keep the chain tension right. I think you're looking at $10 if you do it that way.

Or you can get a SS conversion kit with a cog, spacers, and tensioner. That's like $20 over on ebay.