You must be able to manage a number of things at once. Though it may seem complex at first, with practice & use, it becomes "second nature" and a valuable resource in your ability....to survive. 1) Look though the hole at your target. 2) Find the "dot" in the back mirror (usually on your hand, face, or shirt). 3) While looking at your target, arrange the mirror such that the "dot" crosses back & forth over the hole.
In other words, while looking at your target, align the reflected hole with the real hole, and you're...
Perhaps of Interest:
To check yourself, sight in on a reflector such as a license plate, taillight, or freeway sign. they'll really "light up".
In a 15 to 20 mile range, a 3" or 4" mirror is sufficiently imposing that you not only can see it, you do see it. For greater range & brilliance, use your small mirror as a "sight" by tapping it (in the same plane) to a corner or edge of a larger mirror.
It can be used with Morse code by clamping to a tripod and interrupting the continuous signal with a cardboard deflector at appropriate intervals.
This sighting method works because it lines up the mirror exactly perpendicular to a point halfway between the sun and the target (which is also halfway between the eye and the reflected hole).