Painting with light requires a very steady hand or even better a very sturdy tripod. The Christmas tree is done as a zooming effect – something that can only be done with a SLR and not a cell phone. I have not been able to find an cell phone app that will allow me to manipulate the lens after the shutter has been pressed. Also a very longer shutter may be needed to get the desired effect. By changing the aperture between shots you can vary the thickness of the light. The zooming effect can also be done during the day but you may need a Neutral-density filter to block some of the light. There is another type of light painting, in a very dark area, someone will write something using a flashlight or colored light. Or attach a light to a string and move the light. Some people even have used lit steel-wool (do not do this in a fire prone area and make sure all precautions are used). Use your imagination.
