Turn off the light you intend to use the barn door with and allow the light to cool.
Select a barn door that matches the dimensions of your light. As long as you know the model number of your light, which will be printed on the unit, you can compare it with various barn doors. If your barn door is large enough that it requires a reflector attachment, proceed to the next step.
Loosen the mounting screw on the reflector and slide the small end over the front frame of the light. Tighten the mounting screw to secure the reflector to the light.
Open the barn door flaps and loosen the mounting screws on the rear of the barn door.
Slide the barn door over the reflector. If there is no reflector, you can simply slide the barn door over the front of the light. Make sure the hooks on the front of the barn door are facing down. This means the barn door is right-side up.
Tighten the barn door mounting screws around the frame of the reflector or light.