Camera on tripod at low angle, room lights off, expose for tree lights, shooting at a slow shutter speed to burn them in to the image, use cable release or self timer, white balance set to tungsten. Experiment with turning a room light on for a bit of fill light if you like. This was shot with one room light just left of camera, and it was a torchiere so the light was high and bouncing off the ceiling, with a 3 way bulb on the lowest setting. ISO 400, f4, 1/4 second. I lightened the tree top angel in post processing to give it some glow, added saturation, and applied high pass sharpening.