How to add return air ductwork in the basement.

You will need to answer the following questions before someone can help:
1. What size is the indoor unit? If you don't know just give us the model number.
2. What size is the ductwork currently attached to the return of the unit.
3. What are the floor dimensions of the basement?
4. What size is the house above (or at least the size of the rest of the house that is cooled by this unit you are modifying).
5. Is the unit a heat pump, gas heater, oil heater or electric heater?