Lie on the floor with your knees flexed (feet on the floor) and stick your fingers into the meat of your ass.
Squeeze your ass cheeks together as your method of engaging them  and not by trying to over extend your hips or play with your pelvis; keep that neutral.
Once that is mastered,  bridge the torso off the floor. At this stage, you , a friend or your trainer needs to feel your hamstrings. People who are “hamstring dominant and gluteal deficient will engage the hamstrings prior to moving” (McGill: page 195 Ultimate back Fitness).
This is the wrong pattern. The glutes should drive this action. To help  we can put our foot against your toes, and whilst asking you to squeeze your ass, we  can help your quads engage by lightly cueing from the knees ( so either a finger hook under the knee to gently pull them up, or if you know each other, a quad stroke ( to encourage the hamstrings to switch off) ).
Once you get  your  ass  engaging, “Boom” your back gets  a bit more resilient  and your squat gets better!