2 of the things that need to happen in the Squat:
1 - Knees and hips break at the same time
2 - The knees reach their forward most spot about 1/3 of the way down and stay there throughout the set
So your hips begin to move back first, and then the knees start to bend, causing the knees to slide forward throughout the rep. So you can think about getting the knees forward and out right away(to their spot) as you start to descend and holding them in place through the rep