It's usually difficult to induce changes in brainwave frequency quickly or directly. The most common methods utilize sensory inputs as a means to influence brain activity. The two main senses used are sight and sound.
Sight is relatively simple; a strobing light is pulsed at the frequency desired, and the brain tends to synchronize it's waveform and frequency with the strobe. There used to be a device called the "Dream Machine" back in the 60's or 70's which used this principle. It was essentially a rotating lamp-shade with holes in it that produced a stroboscopic effect.
Sound is more difficult to use. Brainwave frequencies are often within relatively low frequencies (1-20 Hz), which are inaudible to the human ear. In order to overcome this, it was discovered that producing two tones of differing frequencies in each ear created the perception of a third frequency. This third frequency (called a binaural beat) can be described as the difference between the two other frequencies. As an example, if one were to hear 400 Hz in one ear, and 410 Hz in the other, a third pulsing beat would be "heard" at 10 Hz. Scientists studied this binaural effect, and found that brainwave activity seemed to synchronize with the binaural frequency.
There exists software that use both of the principles described above. One of the most widely used is "Brainwave Generator". The overall efficacy of binaural or visual cues to induce certain brainwave activity is still disputed, so I recommend trying it yourself.
There are other methods to induce specific brain activity, such as extra-cranial electromagnetic stimulation, which utilize electromagnetic fields to alter brain activity. This is often difficult to control without the proper knowledge and equipment, and can sometimes induce undesirable effects. However, this method does prove therapeutic for a variety of conditions, including Alzheimer's, depression, bipolar disorder, etc.
Hope that helps!
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