Lie on your back with your hands behind your head, then bring your left elbow to your right knee while straightening your left leg. Alternate sides for a total of 20-30 reps.
The leg raise is a simple but effective exercise for targeting the lower abs. Lie on your back with your legs straight, then raise your legs up to a 90-degree angle and slowly lower them back down.
Begin in a plank position, then shift your weight to your left forearm and raise your right arm up towards the ceiling. Hold for 30 seconds to a minute, then switch sides.