I was using my spare bike as trainer during winter, took off the front and rear derailleurs, shifters and ran the chain directly from the middle chainring (39t) to the cog that gave me the straightest chain on the rear cassette (12-25) so technically it was already a single speed lol.
just cleaning it up by removing the unnecessary chainrings cogs and installing brakes again so that I can ride it outside lol
this is the conversion kit that I bought for the rear
https://www.ebay.ca/itm/202713979506