Lie down on the floor with the bottom of your feet touching each other.
This should bring your knees to the floor.
Your arms are at your sides at the bottom position. As you raise up, they elevate overhead.
Arms are passively at your sides
Take a breath out and imagine your abdominals curling your torso into the top position. Similar in the manner of a bicep curl.
You don't want a rigid torso position. Your back should not be arched during the execution of the exercise.
Your core is not "braced" as if you are doing a squat or deadlift.
From an anatomy and biomechanics perspective, your abs flex your trunk.
You cannot activate a muscle if it is tense.
At the top position, you are "uncurling" your abs back to the floor.
Your neck is kept in a neutral position from start to finish.
Don't hold your breath on the concentric or eccentric portion of the movement.
*Tips*
At first, your legs will want to elevate or "spray" out. Through practice and greater core stability, you will be able to execute the exercise without much hip flexor involvement.
Create the mind-muscle connection your legs are passive throughout the movement. Your quads and hips should not be taxed after each set.
Again, you are "curling" your body using your abdominals to complete the exercise.
Don't throw your elbows forward to elevate your torso.