The positioning of the hips is important in skiiing. They must remain square to the skis so your knees are free to angulate and you can maintain maximum pressure on the skis.
To determine whether your hips are square to the skis, imagine a line through your hips and another across the front of your boots. When these lines are parallel, your hips are square to the skis. In this position, you can drive your knees into the hill and can also center your weight front and back over the skis to maintain maximum pressure (see photo).
If your inside hip is pushed too far forward, your outside knee is blocked. You are not able to angulate the outside knee as necessary to put the ski on edge, and your weight is forced onto the inside ski. Pressure cannot be maintained on the outside ski, so the skis will slide.
If your outside hip is pushed too far forward, your weight is forced onto the front of the skis. The tails of the skis will slide because there is little or no pressure on them.
Hip position is one of the hardest concepts to master for nearly everyone, beginners to experts. Isolating the position of your hips as you ski is difficult and requires discipline and concentration.
Rather than waiting until you're on skis, practice at home without skis. Stand square so you can angulate your outside knee and maintain proper pressure. Practice as if you were turning one way, then as if you were making consecutive turns. As you link the turns together, your hips must rotate slightly.
Once you understand this concept at home, try it on skis. Practice until you feel your skis carve.