Do these using a decline utility bench. Get your chest towards the elevated end of the bench. While holding a dumbbell between your feet, perform leg curls.
This variation is crazy-hard compared to the typical lying leg curl machine. First, the weight is held out at the furthest leverage point – essentially at the ball of your foot versus the ankle or mid-calf as it is with a machine curl.
Second, your feet are in plantar flexion (toes pointing). Because the gastrocnemius muscle attaches across the knee joint, doing a leg curl with your feet in dorsal flexion (toes pull back) makes it easier. So keep your toes pointed on this one, set your ego aside, and really hit your hamstrings hard.