Having the proper ski stance and posture ensures that you stay balanced and in control of your skis. First, you should stand (relatively) tall and spread your feet about shoulder-width apart.
You should flex your ankles and tilt your shins forward while keeping your shoulders slightly in front of your hips. Your weight should be centred over your feet.
Keep your arms slightly out and off to the side, holding both your poles with the tips pointed behind your feet. Also, and we can’t stress this enough, look at where you’re going, not at your skis.
Even though having the proper ski stance might sound a bit convoluted, it’s not that difficult once you get the hang of it. That said, you should keep practising and never downplay the importance of a proper stance.