Remove any the flaps and cut out one side of the box.
Apply adhesive to the backs of the mirrors and place them in the box. For best results, put two to four mirrors inside the box. If the box is large enough, you can put a mirror on each of the three closed sides and one on the bottom, where the box will sit on it.
The mirrored box helps a plant grow because, as light comes through the open side, it shines onto the mirrors and reflects onto the plant, magnifying the light it receives (Image 1). One word of caution however: Too much direct light can burn a plant's leaves. If your window gets a lot of sun, you can use shiny aluminum foil instead of a mirror.
The goal of the mirrored planting box is to give a transitional plant (one that is about to go out to or come in from the garden) the light it needs. If you see that the plant is overheating, you can dull the mirrors by spraying them with hairspray (Image 2) and sprinkling dirt on them to make the surface less reflective.