Lie supine (on your back) on the floor with your arms and legs straight and heels resting on a bench.
Spread your arms out to the sides for stability.
Raise your left heel off the bench.
Exhale as you raise your hips off the floor by extending your right hip.
Inhale as you lower your hips to the starting position by flexing your right hip.
Repeat for the prescribed number of repetitions.
Repeat the exercise with your left leg.
Comments and tips
Do not lock your knee. Keep it very slightly flexed.
The lying single straight-leg hip extension is not an effective exercise for overloading your hamstrings. Do not use it at the gym, where you have far more effective exercises to choose from. Use it only when you do not have access to a gym, such as when you are on vacation.
You can make the exercise more difficult by holding additional weight on your pelvis.