Enter build mode and select the staircase going down icon (this is the basement tool). You'll be asked to drag a box on the ground where the basement will exist. You can then use staircases to connect the two floors. Basements can be decorated just as any other floor of the house. Use Page Down to go down a floor once you've got your basement drawn to access it. I'm doing this from memory but trying to answer your question quickly. I hope this helps.