Position the external angle at least 10mm off the ground. This way if there is any movement on the floor the angle will not crack. Place the external angle against the straightest point of the corner, roughly in the centre. Position this firmly by placing pressure onto the middle of the angle, and pin into place with your staple gun. To check the angle is straight all the way down, place your straight edge alongside the angle, adding pressure. Once this is straight, staple the remainder of the angle into place.