The main rule when using these mirrors is that they should never, ever be used indoors.
The second Feng Shui rule when using the Bagua mirror is to use it only when there is negative Feng Shui energy – Sha Chi or Si Chi – in the direction of your home or business address. The Feng Shui Bagua mirror is not intended as a decorative piece and should never be used as such.

If you find yourself in the situation where you need subtle protection in your home or office from the potentially damaging effects of Sha Chi energies (for example, sharp or aggressive-looking structures or buildings pointing in the direction of your home or workplace) you might choose to use the Bagua mirror. It is best to ask an experienced Feng Shui consultant how best to place the mirror. Traditionally, the mirror is hung above the front door, or above a large window in the direction of the negative objects and energies.

Outdoor use: If possible, hang the mirror in a sheltered place to protect it from the weather.