Identify the cold cylinder as described by backpacker (saves a lot of time and effort). Bring a spare plug with you if you have one, Trace the leads back from the problem cylinder to the mags, and work out which lead top or bottom goes to the rough mag.This will identify which is the duff plug.
Try cleaning the plug first. If no joy change it. if that does not work change plugs top for bottom, if it is a problem lead you will still have the rough running on the same mag, and probably need an engineer.
With an aircraft on private category changing of plugs is allowed under pilot maintenance.