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.
Drive through your heels, engage your glutes 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, Lower back to the start, & repeat for repetitions.
At no point should you feel strain in your lower back.