I converted a Schwinn '10-speed' bicycle to a Singlespeed.
My rear wheel came with a Hyperglide Cassette Hub. This hub uses cogs that have inner teeth that mesh with grooves on the hub. A locknut keeps the cogs from sliding off sideways. I removed the cogs, slid a tubing spacer ring onto the hub followed by one cog and then another ring and finally the nut. Singlespeed!
(More Singlespeed construction details are available all over the web.)
I liked it!
Just ride and forget all that gear shifting stuff.
I then thought I might want to try a fixie.
I needed a hub that was without a freewheel.
I didn't want to spend any money on a new wheel with a track (fixed) hub.
I didn't want to buy a new fixed hub and lace it onto my wheel.
I decided to hack my existing freewheel.
I fixed it!