Trying Entrainment - How's it done?

The two primary methods are the use of flashing lights or sound with a beat at the desired frequency. With the lights, this is usually accomplished by using special glasses, much like opaque sunglasses with lights built into them.

The factors you can alter with the goggles is the flash frequency, the brilliance, whether both eyes flash together or independently, and how fast the lights come on (are they simply on or off or do they smoothly increase from off to dim to bright and back again.

With sound there are many more variables. The frequencies involved can't really be heard - they're too deep - they're more felt than heard. Remember the range we're dealing with is 1.5-20Hz and the lowest pedal on an organ is 16Hz which feels like a deep rumbly vibration, more felt than heard. Therefore we can't simply use the correct tones as most speakers and headphones couldn't reproduce them and even if they could, our ears wouldn't hear the tones anyway. There are three solutions to this problem - pulsing bursts of noise, binaural beats and modulated noise, as used by the Brainwave Generator and CoolEdit (these are explained in more detail later). The first two of these are the simplest to explain. 

Pulsating bursts of sound are self explanatory, except that the sound itself can have a secondary effect. The sound can either be noise which gives a fast chuff-chuff-chuff effect or it can be a tone that is being pulsed. In that later case, the frequency of the tone can have some important effects. For more information on this see James Mann's book "Awakening Mind I" which unfortunately no longer seems to be available.

The actual tones themselves an be pure tones (sinewaves) or be rich in harmonics as are the sounds of different musical instruments and the human voice. Each of these appears to have a different effect. A well known example  of this (at least in New Zealand and Australia) is the hypnotic sound of the Aboriginal Australian dijeridoo.

The discussion above implies that only one effect is used at once, based on the idea that the brain is in one state at any one time. This is a gross simplification, the actual EEG measurements depend on where on the head you attach the electrodes (which part of the brain you're measuring). In reality, the dominant frequency is exactly that - the loudest, but the secondary effects are nevertheless real and significant. To this end, both complex tones rather than pure tones (sinewaves) can be used, and multiple sets of frequencies are often used simultaneously, enabling a large variety of effects to be achieved.