Yours is an in ground pool, right???

Break the return line between the filter output and the pool. That is where your heater goes. If you have a chlorinator ... it should be between the heater output and the pool (chlorine is the last thing to go into the water before the pool). You will need a fuel line of appropriate size for the heater and either 110 or 220 volts of power. Your heater shold have a safety built in that prevents its operation unless there is water pressure.