Get on your knees and anchor the back of your heels under a bench or a barbell. Ideally, for reasons explained below, in Comments and tips, your feet should be fully dorsally flexed (i.e. your toes should be pointing up to your shins).
For reasons explained below, flex your hips a little.
Cross your arms over your chest (or you can keep them by your sides).
Inhale as you allow your body to slowly fall forward, controlling the descent as much as you can with your hamstrings.
At the bottom of the movement, catch your body with your arms and lower your torso to the floor as if performing a knee push-up.
Exhale as you push your torso back up and off the floor just enough until your hamstrings can take over and pull your body back up to a vertical position.
Repeat for the prescribed number of repetitions.
Comments and tips
Use your arms as minimally as possible. Their role in this exercise should only be to prevent you from hitting your face on the floor. Your hamstrings should be doing as much of the work as possible.
The leg-curling section of the inverse leg curl is an isolation movement with a pulling force, whereas the push-up section is a compound movement with a pushing force. Since the use of your arms will be minimal when performed correctly, I categorized this exercise as an isolated pull.
Keeping your feet dorsally flexed will maximize the contribution of your gastrocnemius.
During the push-up section of the inverse leg curl, keep your elbows slightly tucked into your body (as you should be doing whenever you perform a horizontal compound pushing exercise, such as the push-up). This will reduce the pressure on your shoulder joints.
Make sure the bench or barbell under which your heels are anchored is secure. You can also just get someone to hold your feet down.
To protect your knees, use a mat.
The inverse leg curl is also known as the manual hamstring curl, bodyweight hamstring curl, and the Norwegian or Nordic hamstring curl.
Use this exercise only when you do not have access to a leg curl machine.