In QuickScale, open a drawing.
When you first open a new drawing in QuickScale, a red banner appears at the top of the drawing to tell you that the drawing's scale has not yet been set:

This is the first in a "traffic-lights" sequence that takes you through the simple calibration process: red means "not scaled"; amber means "provisionally scaled" and green means "scaled".

As the scale of the drawing is unknown, we need to set the scale by drawing a calibration line using a known accurate site measurement. If the drawing does not contain any measurements you can use any item for which a measurement is known (e.g. door width, window width, etc.)
Navigate to that part of the drawing where you are going to draw your known measurement (zoom in to get a good clear view of the part of the drawing to be measured) and click the calibration tool button:

Now click one end of the measurement and the move the mouse to the other end and click:

The calibration dialog will now appear:

Enter the exact dimension or just click OK to confirm the measurement. The "traffic light" banner will turn amber to indicate that we now have a provisional scale.
Repeat the "Draw a calibration line" process using another known measurement. The "traffic light" banner should now turn green to indicate that we now have a confirmed scale.
That's it. You should save the calibrated drawing now and you are ready to take-off all the measurements you need or print the drawing to scale.