Find an anchor point for your hands. You can use a fixed bar / post & have your hands above your head. Or lay on on a bench and grip the sides of the bench,.
Your head should rest on the floor and you should avoid feeling any pressure in your neck (Try adjusting your hand position and your distance from the anchor point to help with this)
Bend your knees and roll up on to your shoulders, create a stable base by retracting and depressing your scapula (Push your shoulders blades down and into the floor, so you are not resting on your spine)
From here, extend your legs up to the sky. Squeeze your glutes, and extend your hips all the way through. Forming a straight line from your shoulders to your toes. Squeeze at the top, and bend the legs to return to the start position.
Stay hollow, maintain PPT (Tuck tailbone, core engage)
Point elbow forward, don't allow them to flare out.
Contract your torso, bend your legs and drive your legs up as if you are performing a reverse crunch, but keep your body rigid from your shoulders to toes, and avoid bending the hips. Point your toes to help maintain proper body alignment.
Slowly straighten your legs while maintaining previous position. Hold for a second and get back to starting position.