Stand with your feet about shoulder width apart, neutral spine, and a slight bend in your knees.
Initiate the movement by hinging at the hips, feel your hamstrings engage, and push your hips back.
(You can use the edges of your hands for feedback, by pushing them into the crease of your hips.)
Keep your spine neutral, lead the movement with your hips. Your knees can bend further to allow you deeper into the hinge.
Continue until you reach the end of your hamstring flexibility, but do not try to go lower by flexing the lumbar spine, your back should remain neutral throughout the movement.
Push through your heels to drive your hips through. Fully extending at the top, squeezing the glutes. Repeat for repetitions.
Key point is to keep the spine neutral - A good way to tell if your back is rounding, is loss hamstring engagement.