Computer paper with black ink is the tinder of choice for practicing this technique because anyone should have access to it, and it's fairly consistent and reliable. The bottle needs to be filled with water, and ideally should have no air bubbles inside. By turning the bottle upside down and using the rounded convex part at the top, we can use a water bottle as a make-shift magnifying lens. The curved water refracts the sunlight, and give the ability to focus it. The closer the rounded lens is to resembling a liquid sphere, the better it will work. If bubbled are trapped in the bottle, they will distort the sunlight and weaken the intensity of the beam.
This technique isn't easy, and takes a lot of practice to master, so don't wait until you're in a survival situation to try it.