I do this all the time. it really helps burn the upper abs.
1. Lay on your back, with knees up and bent like you’re about to do a regular crunch or sit up
2. with your legs still hip-width apart, just bring your knees together
3. with hands behind your head or on your chest, curl/crunch up
start out by bring your neck off the floor, then shoulder blades. all in all, it’ll burn after 30-60 seconds if you’re doing it correctly :)