In Chaos Magic theory, a sigil is a representation of a desire used to implant that desire into the subconscious mind. The technique of sigilization can be thought of as a generalized form of spellcasting. It is similar the Satanic Greater Magic described in The Satanic Bible. However, while the target of Satanic magic is another person, the target of sigilization is (generally) oneself.

The method given by Phil Hine in his booklet Oven Ready Chaos (available online) is abbreviated SPLIFF. This stands for

Statement of Intent
State as simply as possible what you would like to achieve. E.g., "It is my will to have a Sony Clie."
Pathways available
Mundane action is not to be ignored. Your goal will be more likely to manifest if give the magic a helping hand.
Link intent
Form the intent into a glyph or other representation. Common ways of doing this are knocking out the repeating letters from the SoI and combining the remaining ones into a picture, doing the same but forming them into a mantra to be chanted, and creating a picture of a desire which you form into a glyph. The final sigil shouldn't resemble the orignal representation.
Intense gnosis/indifferent vacuity
By excitatory (dancing, chanting, sex, etc.) or inhibitory (meditation, hypnosis, concentration, etc.) methods, one passes the sigil to one's unconscious mind. Gnosis, as referred to in chaos magic, is not that of the Gnostics, but rather a state in which the subconscious is receptive to programming, also called psychodrama.
Fire
The sigil is said to be implanted at the peak of gnosis, i.e. orgasm, deep trance, interruption.
Forget
Forget your sigil. This is to prevent you from interfering with the spell once it been cast. You don't necessarily need to forget the desire, or that you did a working for it, but you certainly shouldn't obsess over it. Do your thing and let the magick do its.

Sigilization is thought to have been pioneered by Austin Osman Spare, but it may have been used as well by the Freemasons. It was popularized by the IOT. This technique is a basis for another popular technique used by Chaos Mages, the creation of servitors. Also popularized by the IOT, a servitor is a spirit who does the magician's bidding.

One last thing: While the basic technique calls for implantation in one's own subconscious, sigils can also be implanted in those of others, as is shown by corporate logos.