Lie on the floor, head neutral, hands on the floor. Bend your knees and 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, and extend your hips.
Shoulder, hips and knees should form a straight line. Avoid extending your lower back, the prime movers should be glutes and hamstrings. Squeeze at the top. Lower back to the start and repeat for repetitions.
At no point should you feel strain in your lower back.