Start the sign for basement by taking your non-dominant hand and laying it flat and horizontal in front of your body, as if making a floor with your hand. Make your dominant hand into a fist with your thumb extended upward, and move your hand in small, horizontal circles under your non-dominant hand, as if you are drawing attention to the basement space under the floor.
If you have a basement play room, make the basement sign every time you go down to the space. As your baby gets more advanced, you can talk about a person or a toy being down in the basement. “Dad is downstairs in the basement.”
Babies love learning about the names for different rooms. If you have a basement, you can teach this sign along with the other room signs.