Lie on the floor, head neutral, arms by your sides. Bend your knees, position your feet flat on the ground, close to your glutes. Shins should be close to vertical. Bend one knee and lift towards you chest, so you can interlock you fingers around the leg.
Drive through your heel, engage your glute and hamstrings, to extend your hips.
At the top shoulders, hips and knees should form a straight line. Avoid extending your lower back, the prime movers should be glutes and hamstrings and not the lower back. Squeeze at the top, before lowering to the start. Repeat for repetitions & switch sides.
At no point should you feel strain in your lower back.