The Flashlight Method
Nearly all flashlights use a parabolic mirror to magnify the light emitted by their bulbs. In fact, the bigger the bulb, the bigger the mirror, usually, and we want the biggest mirror we can get.
Take apart your flashlight and remove the bulb. The bulb, it just so happens, is designed to sit at the focal point of the mirror. Simply insert your tinder through the opening for the bulb, and aim the mirror up at the sun. The mirror will focus the rays onto the tinder, then wham-o, you have the beginnings of a fire.
Although I typically recommend LED flashlights because of their efficiency, their smaller reflectors don’t work as well for this method.