First, some tankless systems do not allow recirculation (and if used, voids the warranty), so that is the first hurdle. If allowed, then you must follow the specific setup dicated in their manual. Retrofit recirculation systems can use the cold water line with a crossover to get the hot to the end of the line. This means that the cold line is partially, if not fully, full of at least warm water. Some people find this particularly annoying, but without a dedicated return line, is one of the things that just is.
A recirc system will have at least three parts, the pump, a crossover (if not using a dedicated return line), and a check valve. If you do a forum search, you'll find lots of discussions on them.
The pump on most goes in the utility room near the heater. One retrofit unit (that I have in my townhouse) is made by RedyTemp. Lang, Chilipepper, OnDemand, and others have units.