To take a Christmas tree silhouette picture, you'll need to set up your camera in front of the tree in a dark room with no other lights on. Make sure to use a tripod or set your camera on a steady surface so that you can take a long exposure shot. To get the best silhouette effect, set your camera's exposure time to at least 30 seconds. You may also want to use a remote shutter release so that you don't shake the camera when you take the picture. Once you have everything set up, take the shot and wait for the magic to happen!