In self-diagnosis, you are both "doctor" and "patient".
After setting the mirror toggle of the Snellen Chart, you are standing in front of a mirror. For example, setting the Distance to 2 meters, your eyes should be 1 meter away from the mirror, while your phone/tablet is standing near one of your ears for example the left ear for the left eye, and cover the right eye with an “ eye patch”.
The mirror toggle is available only to Snellen Chart.
You should be able to use Tumbling E Chart and Landolt C in front of a mirror naturally. For example, the E on the screen has limbs pointing right, and the E reflected in the mirror has limbs pointing left, you may swipe right on the screen to mark a correct answer, and this action is mirrored as swiping left in the mirror.