Kneeling chairs work by shifting the angle your body sits and where you place your weight. Your weight is still on your bottom when sitting, but the weight is placed on the shins when you shift or change positions. Therefore, it is essential that most of your weight is on your hips and that only about 10-15% is on your shins. Also, your kneecaps should not touch the padding; this will help your back be more aligned and balanced.
Keep in mind that there will be an adjustment period with your kneeling chair (especially when it comes to the new pressure you're putting on your shins), so don't start sitting on it for eight hours a day. Instead, see how you feel after sitting on it for an hour or two and switch back to your regular chair. You can work your way up to using your kneeling chair for an appropriate and comfortable time.
No matter what chair you are sitting on, it's recommended to take a short walk and stretch your back once every hour.