This conversion technique requires a lot of time, so it's not for everybody. First you take any multi-geared, free wheelin' bicycle - set it outside in your backyard for a decade or two. Eventually that freewheel and those deraileurs will lock themselves up tight from wholly natural causes.
You remove the deraileurs and brakes, and you're set - just take a look at this natural conversion I carried out myself (project was started unintentionally by the previous owner).
And here's the obligatory crown clearance shot...
Keeping the five cogs in back means I can select five different ratios just by loosening the rear wheel and putting the chain on a different gear.
I should note that if you intend to ride on pavement instead of grass, tires are recomended - unless you can stand the noise.