Hip thrust

Sit on the floor with your upper back (just below the shoulder blades) resting against the side of a stable bench.
Roll a padded barbell over your legs until it sits in the crease of your hips. Plant your feet firmly on the floor, about shoulder-width apart, so that your shins are vertical when your hips are lifted.
Keeping your chin tucked, drive through your heels and squeeze your glutes to lift your hips toward the ceiling. Your body should form a straight line from your shoulders to your knees at the top.
Pause for a moment at the top, focusing on the glute contraction, before lowering your hips back down with control to complete the rep.

Tips

Position your feet so that your shins are close to vertical when your hips are fully raised, which keeps the drive through your heels direct and your knee angle consistent throughout the lift.
At the top of each rep, pause briefly with your hips level and your glutes actively squeezed before lowering with control.
Keep your chin tucked throughout the movement so your head stays in line with your spine rather than tilting back as your hips rise.

Letting the hips rise higher than the shoulders-to-knees line causes the lower back to hyperextend at the top instead of the glutes completing the movement.
Placing the feet too far forward causes the shins to angle away from vertical at the top, shifting the drive away from the heels and reducing the height the hips can reach.
Allowing the knees to cave inward during the lift means the legs are not tracking over the feet, which disrupts the stable base the movement requires.