Imagine curling the bottom of your sternum towards your hips.
Your abs should feel as if they are being sucked inward.
Arms can elevate as if your hands are holding a small plate or dumbbell.
At the top, your upper back is off the floor
Uncurl from the top back to the starting position slowly.
Breathe out at the top and as you descend, again, feel your abs being sucked inward.
*Tips*
Keep your legs passive throughout the movement. Using your lower half with activate your hip flexors which will negate your abdominal musculature.
At first, your legs will want to elevate or "spray" out. Through practice and greater core stability, you will be able to execute the crunch without much hip flexor involvement.
You are "curling" your body using your abdominals to complete the exercise.