To create a basement, click on the Manage/add levels icon.
Then click Add floor.
Select Create a floor > Below GF.
You can choose to duplicate your walls, doors, windows, and openings as well as the ceiling height and slab height.
Tip: do not duplicate doors, windows, and openings, the majority are on the outside and are not useful to you.
Note: if you have a floor above your ground floor that is larger than the ground floor itself (as below), it is possible that the walls of your basement go up to the first floor (as below).
To bring the basement walls down, click on one of them and click Properties. In the menu that appears, deselect Up to ceiling. Repeat this for all the excess walls on your ground floor.
You now have a basement!