Make sure you are on the Building Tool:
By default, the Create items automatically option is enabled in the Drawing settings.
Then, sketch the outline of the proposed building
Once you have drawn a "closed" shape (the final stroke indicated by the yellow arrow above), Codesign will automatically create a Building.
A Building will be created based on the current Default Settings.
If you have Create items automatically disabled, you can create a Building by drawing the outline and then using your finger to tap inside the enclosed shape.
This process allows you to sketch continuously without a building being created until you are ready - refer to Sketching Tips for more information.
With the Usage Palette open you can also drag a specific Usage Type, and drop it within the enclosed shape. Again, a Building is created based on the current Default Settings along with setting the Building Usage based on the Usage Type that you dragged.