I’d start at stage 1 with them.
Squat therapy can be a starting place.
Rebuild their technique if there are deviations.
Addressing both aspects to take a more complete approach.
Setting correct positioning before movement begins sets the person up for success..
Creating torque through the correct joints and load ordering are key.
Box squatting can be an option.
Addressing the soft tissue and muscle imbalances allows the body to move properly.
If the person does not understand the movement they will have a harder time performing it correctly.