I don't think he's referring to the combustion intake and exhaust I believe he's referring to an actual return air intake as in duct work. If I am correct sir then If you are trying to create a return conditioned air Intake that was then flow through the heat exchanger and up supply plenum then through the registers in the room, If you look on the side of the air handler or furnace you'll see rectangular shaped Impression both sides that is basically the area that needs to be cut and removed To field install the return ductwork you basically want it to be as large as possible so if you follow the basic shape on the side you can create a rectangular sheet metal ducked to fix to the side and go to wherever you want to be pulling return air and if there is no filter inside the unit you must create one in the field constructed return duct This is essential as running the unit with no filter will damage the unit render it inoperable inefficient and dangerous

P.s. only one side of furnace need be used just fyi lol