There are multiple methods for entering a trance state. This is understandable, because proper trance is required for a number of things. I will be detailing a method called "the free fall method". I prefer this to overloading and rocking, simply because it's a very direct technique.
Step 1: Adjust yourself into a position where your completely comfortable. You can be in a comfy chair, or laying down in bed, it doesn't matter. All that matters is that your current position will not need to be adjusted to comfort yourself again. This also includes temperature, be sure to be in an area where the temperature wont fluctuate badly.
Step 2: Focus on your frontal cortex, this is the front of your brain that is in charge of conscious thought and processing. The objective of trance is to sink your brain activity closer to the middle of your brain. In some cases to the back of your brain for deep trances. During focus, you should come to a point where your the intensity of your being is located at the frontal cortex.
Step 3: Imagine yourself falling slowly with your back facing the direction your falling. While envisioning this, try your best to feel it happening. The more you feel the shift, the more effective this meditation will be. When your comfortable with your decent, try to imagine yourself falling faster and faster. Don't fall to fast though, or you might snap out of the trance. If you become tense, stop the exercise, relax and continue.
Step 4: When you have reached the point where your consciousness is in the middle of your head or at the location required, then you must relax your thoughts and clear your mind. You will now be able to begin the required task that you needed the trance for.
Usually the deeper the trance the more effective the result. When returning from a trance, you must slowly reanimate yourself, depending on the intensity of the trance.
It is often that trances will make you unable to feel your body, and noises will be sharp and painful. Choose a meditation area that suits to complement comfort. Make sure you wont be bothered for the duration of the exercise.