Many practices in Magick require you to first be in a trance state, particularly the more clairvoyant abilities, as being in a trance will help in developing these abilities. Think of the mind as a kind of muscle, the more you develop it the more stronger it gets and keeping up with regular training keeps it perfected. This is what a trance does, it trains the brain. Before starting make sure in a comfortable place and wont be disturbed.
The easiest technique is the ladder method, especially for novices but even then practice is required. Imagine yourself on a ladder in a completely pitch black room, feel yourself standing on a rung. As you breathe in feel yourself descend a rung or two and as you exhale you stand still. Keep doing this until you feel yourself completely relaxed, then when it feels right let go of the ladder and let yourself fall backwards into nothingness, falling further and further down. The feeling of falling and the total relaxed state will put you into a trance.
Another technique, made famous by E A Koetting is the Trataka method or the Theta Gamma sync. To do this stare at a spot on a wall with your eyes opened but relaxed. Let your thoughts drift away to nothing, and eventually you will see a sort of rain in front of your eyes. Switch to another point just off of where you were looking and you will be in the Theta Gamma sync or Rapture state as E A Koetting calls it. This is also a trance.
If at any point you feel dizzy, bring all your attention to the front of your body, which will snap you out of it . However dizziness and some pain may reside. Whilst in the trance, all other senses are heightened and coming out too quickly or being snapped out of a trance can lead to headaches and pain that lasts quite a while.