Right way to plumb tankless w/h?

I've only installed one tankless water heater so don't know much about them. Is there truly a "right way" to plumb a tankless water heater? Should there be an expansion tank, check valve, pressure gauge, etc.???

tankless heater does NOT have any stored water and it only heats while water is flowing, so it does not NEED an expansion tank, or check valve, and few even have provision for a temperature relief valve, although all of them come with a pressure relief valve. None of these are needed IF the heater is working properly, but then, safety devices are installed to take care of problems when things DO NOT operate properly.

Find an instalation class in your your area for Noritz or Rinnai. They are usually free and are just what you need.