How to install tankless water heater?

Just in-line nearest possible and conveniant to the outlet.
It all depends for what use you like to install the system, and what specific type you use.
I have installed many in-line heaters in showers, just under the shower head.
And in kitchens under the sink in a cupboard for a warm water tap.

Are you talikng about a whole house tankless heater? If so they can be tricky depending on fuel type, venting and piping. What brand di you buy and where did the 25 page installation manual wander off to?