This is the technique:
Make sure the bottle doesn´t have a screw cap <img src="images/graemlins/wink.gif" alt="" />.
Hold the lighter in your right hand. The bottom should stick out between thumb and index finger. Grab the bottle neck with your left hand. The lighter should fit tightly between the left index finger and the cap. Put the lighter in a slight upward angle between the index finger and the cap. The upper edge MUST have good contact to the cap. Now tighten your grip on the bottle and rotate the lighter downward (it is used as a lever). The cap should pop off. It sounds more complicated than it is.
Note:
For your first attempts you should take an almost empty lighter as it´s quite easy to break into the gas tank.