How do you properly install flashing on a roof to ensure it is secure and watertight?

To properly install flashing on a roof, you should first clean the area where the flashing will be placed. Next, apply a layer of roofing cement to the area and secure the flashing in place. Make sure the flashing overlaps properly and is sealed tightly to prevent water from seeping in. Finally, inspect the flashing regularly to ensure it remains secure and watertight.