At 12:17 AM, on any given night, arises the opportunity to awaken an alternate soul. The most common way of viewing them? Through a mirror.
It is through said medium that the process must take place. Begin at exactly midnight. By no light but that of a single candle, stand before the selected mirror.
For ten minutes you must concentrate in silence, focused entirely on your reflection. Do not look away from the eyes, for it will be interpreted as weakness and you will be overcome.
After ten minutes have passed you must draw blood to smear in a line across the eyes of your reflection. Doing so will blind it, and you will watch as your own features begin to warp.
Slowly, gradually, they will mutate into a frightening creature, one beyond the comprehension of those who have not experienced it. You must not look away through the entirety of the change.
Soon the writhing movements of the image will cease. By now an echoing, inhuman sound will resound all around you, the creature will begin to ease toward the mirror's glass. You must keep watching as it approaches.
If you do not extinguish the candle at exactly 12:17, the creature will escape.
Be warned, should you succeed; through any polished surface, be it mirror, wood, or window, your reflection will always be watching.