This is more or less in every abs exercise routine so most of you might already know how to do the criss crosses. Here is what you do. You lay on your back, take your hand being your head for support. Don't lock your fingers you will start pulling your head up.
Now raise your head up along with your shoulders and arms. At the same time, raise your legs about 6 inches off the ground. Twist your torso to the right so that left elbow is now at the front and your left elbow is at the back. At the very same time bring your right leg in towards the chest and try to touch your left elbow to your right knee.
Now go back to the starting position (i.e. head and legs both in the air) and do the exact opposite. A very good thing about this exercise is that you do not have to stick to a slow movement. You can vary your speed whenever you want.