I need instructions how to install 24 volt transformer in air handler

Well, since there are just a few different air handlers why not tell us what unit you have, and what it is you are trying to wire?

Controls?
Thermostat?

It should already have 1, replacement transformer may have different taps for different primary and secondary voltages,
Most common is use the leads for 240 volts in and 24 volts out, if multitaped transformer.