Then click on the "Add Polygon" button. After doing so, your cursor will turn into a crosshair allowing you to start drawing the building outlines.

Click each corner of the building. As you do, QGIS will create a polygon of the building based on your clicks.

Click each corner of the building. As you do, QGIS will create a polygon vector of the building based on your points of reference.

Right click to finish the polygon. QGIS will automatically ask for an ID number so it can keep track of your shapes (we'll add their real names later!).

Number the building something unique, click OK, then move to the next building repeating the previous tracing step.

After creating the new vector, be sure to add a unique ID number, as you did in the previous step.

QGIS Digitizing Tutorial 4: Tracing Features