How do you install blower motor in a gas furnace?

The same way you took it out.

First you need to de-energize the blower motor by turning the correct electrical breaker off. Most newer furnace blowers are what you call a squirrel cage blower. That means it is not belt driven like the older blowers. You need to label,then disconnect the wires going from the blower, to the control unit. Then you need to remove the mounting bolts from the blower frame. The blower unit should slide or lift out. Make sure your replacement blower is the same horsepower and has the same frame size as your old unit. Good luck