Assuming you have a double chainring set up, an even easier, but not very elegant way to convert is to just shorten the chain and go directly from the chainring to a cog in back, bypassing the rear der.

If you go with the small CR and the 15 or 16 tooth cog on the back, that should give you a gear in the low 70 inches, which is a good around town gear.

Chainline may not be that great, but it should work.