A laser beam is a narrow, coherent light beam created by a process called "stimulated emission." "Laser" is actually an acronym which stands for Light Amplification by Stimulated Emission of Radiation. In a laser, a power supply excites the atoms in a medium such as carbon dioxide or sodium. These excited atoms emit a unique kind of light that is extremely coherent and is of a very high spectral purity. Lasers are known for their coherence. While the light emitted from a flashlight, for example, scatters quickly through space, a laser beam remains tightly focused for great distances. Though creating a laser beam is very difficult and possibly too expensive for the average hobbyist, it is theoretically rather simple.
Place the two mirrors on the ends of the glass cylinder, with the reflective sides facing the interior of the cylinder. This is the "optical resonator" which produces the laser beam.
Fill the cylinder with your choice of gain medium. There are literally hundreds, if not thousands, of choices for the gain medium, from carbon dioxide to helium-neon to argon. For the home-built laser enthusiast, a gas laser is much easier to design and build than a laser constructed around a solid matrix such as a ruby laser.
Install the flash lamp next to the optical resonator.
Wire the power supply to the flash lamp.
Energize the flash lamp for a short burst. The light from the lamp will excite the atoms in the gain medium, stimulating them and making them go from a lower energy state to a higher energy state. These "excited" atoms will emit light, which will bounce back and forth many hundreds or thousands of times from one mirror to the other. Once the light has become powerful enough by virtue of being amplified by many trips through the gain medium, it will exit the optical resonator through the partially transparent mirror, producing the laser beam itself.
Always use eye protection when working around lasers. A laser can easily burn your retinas and blind you, and the higher power lasers can burn the skin as well.