Neutral spine is basically how you are when you are standing. Ideal posture is defined as your ear in line with your shoulder AC joint, through the hip joint, down through the knot of the laces of your shoes. Your breastbone and pubic bone should be in line.
To learn how to keep that position when you squat, use a broomstick as the girls above are doing. The 3 points of contact are:
1. The back if the head
2. The bra-strap region
3. The tailbone (coccyx)
When you move for a Deadlift or squat, you should be able to maintain the position of the broomstick on those points and prevent excessive sliding. All 3 points of contact should NOT move.
Try it and let me know how you go.