Bring your heels closer together. Let your knees travel further forward at the start of the squat. Think of getting them out. Do you see how far they go when you hit the bottom and they slide forward? That's where they should go at the beginning. We don't want that movement hhappening at the end.
But, really, I think the first step is to bring your heels in and point your toes a little more out.
Oh, and hit the buffet line.