Light stencils are an easy way to add in graphic elements to long exposure shots. Basically, this is fired in the frame while the shutter is open during a long exposure. The graphic remains while the person and box is completely invisible.
Watch the vid and make a box as directed. Using any illustration program print out your design onto fine grade paper which is slightly heavier than regular copy/printing paper. I print 2 and paste them together but make sure they are perfectly aligned by holding them up against a bright light source. I laminate mine so they can be reused as they often rip when removed or changed but this is not a necessary step.
I usually set the flash to 1/8 which works well with the settings I use on my camera for light @painting. You may need to adjust this and I suggest doing a test shot before your light work to check the exposure of the stencil. Happy shooting!