The sign for sofa is made by pointing the index and middle fingers of your non-dominant hand sideways and placing the same fingers of your dominant hand on top of the former, as if your non-dominant hand is a chair. Complete the sign by pulling your dominant hand away from the other hand, indicating a "long" chair.
This is a very easy sign to teach at home during family time, or when watching television.
Most of us have a sofa at home, and it is the central place for relaxation. Teach sofa contextually and also by showing your baby different sofas outside the home.