There are numerous 'kits' that contain (nearly) everything you need. If you don't have a dedicated return line, then you use the cold water line. This will cause the water in that line to get warm (to hot) depending on the settings. It gets purged out when you draw cold. Some put the pump at (near) the WH, and some put it under a sink furthest from the WH (I have one of these installed - a Redy-temp unit). All systems require a check valve and some sort of control valve(s) at the sink(s) to create a cross-over to allow the line to be purged. The unit I have has it all in the box, and takes about 10-minutes to install IF you have a place to plug it in. You can have the thing run on a timer, a proximity sensor, or a momentary button, or continuously. A dedicated return line is often plumbed into the drain line at the bottom of the WH. At a minimum, the system requires a potable water pump (brass or SS, not an iron one) and a checkvalve (and cross-over, if using the cold line). The better retrofit units all have some sort of thermostatically controlled cross-over valve underneath the sink(s) you want to have the hot water sooner.