Assume a prone position on a bench, with your legs off the end, and your knees and hips bent to 90 degrees.
Extend the legs outward by engaging the glutes and hamstrings, keeping your back neutral and your upper body static. At the top of the movement your legs will be level with you upper body, avoid lifting any higher as this will place unnecessary stress on your lower back.
Squeeze at the top and return to the start. Repeat for reps.