A sigil may be a picture, a mantra to be intoned, or a picture made from words. The idea is always to take a desire, put it into a picture or a word, then disguise it to sneak it past your conscious mind and embed it deep into your subconscious, to manifest it in reality (what exactly reality is, is a matter to be discussed elsewhere).
After being created, the sigil is then charged (or more accurately, loaded into your subconscious mind the way you load a program into a computer's memory), normally during a ritual that may or may not have all the usual pomp and ceremony (candles, incense, banishing rituals). Sigils can be charged by many different methods such as breath control, sensory deprivation, yoga and sex. Always the aim is to create an altered state (called "gnosis").
Please see the reading list and links for more info.