Hyperextensions are best done on a hyperextension bench, but can be done on a bed or ordinary bench with something (or someone) holding down your ankles.
Lie face down, with your hands touching the sides of your head and your body draped over the edge of the bench. Make sure your hips are supported so your pelvis can't move. Slowly raise your torso to the horizontal position, but no higher.
Keep your head, shoulders and upper back arched through the whole movement.
Try to do a couple of sets af around 12 reps after each ab routine or after each back routine. Don't exercise your lower back more than about three times a week. Don't exercise it if it's still sore from the previous workout.