How do you install a booster fan on a furnace?
You should not need a booster fan on a furnace. If the duct system is properly designed there will be no need for the booster fan. If the system is not designed properly, you would be better off replacing it with a properly designed system.
Actually, this doesn't answer the question. There is no way to know if he/she needs a booster fan without looking at the system. You can get very clear directions with photos at the following site: http://www.diynetwork.com/diy/he_heating_cooling_systems/article/0,,DIY_13894_2274829,00.htm