We have a few things going on.
It looks like you are not centered on your feet to start.
Make sure you root the feet with the weight evenly distributed on:
big toe
pinky toe
heel
Then, I see a forward dump and a knee hinge.
Try to put your back in to the bar as you descend and drive the knees out as you sit back over your heels.
Watch this video, specifically around 1:45 to see how to put your back into the bar.