Making fire with a mirror is something that shows up on TV or in books on occasion, but it is actually possible to do, with patience and the right supplies. A mirror is not the easiest thing to make fire with, but in a pinch, it will create the right conditions for fire to start and build. Realize that fire requires three things to exist: heat, oxygen and fuel. Without any one of these elements, fire will not start or sustain. The mirror provides the heat.
Use a mirror to shine a concentrated beam of sunlight onto a fuel source, such as dried grass or twigs. Hold the grass or twigs in one hand, and aim the mirror underneath in such a way that it catches the sun's rays and directs them onto the fuel source.
Use two mirrors if you are not comfortable holding the fuel source in your hand while you make fire with a mirror. Place the twigs or grass on a noncombustible surface, such as a sidewalk, aim one mirror at the sun and place the other mirror over it to reflect the sun's rays downward. You may have to experiment with positioning, or get help from someone else to hold a mirror.
Watch carefully for smoke coming from the fuel. This is your signal that you are about to make fire with a mirror. Continue to aim the mirror until there is enough flame present to sustain itself. Add fuel slowly to help the fire grow.