Criss Cross is one of the fundamental exercises I almost always require in my Pilates classes. It's a great way to target the entire core, especially the obliques, which can be a hard muscle group to work.
For criss cross, begin with a neutral spine on the mat. Supporting the head with your hands, lift it off the mat while bringing the knees to a 90 degree angle.
From here, you'll cross the shoulder to the opposite knee while lengthening one leg. One leg comes in, one leg goes out. With each exhale, your legs move and your shoulders cross up and over the torso.
It's important to keep the elbows wide and the neck long. Attempting to bring the shoulder across the body, rather than the elbow, will increase the effect on your core. It will take more abdominal strength than if your elbows just flopped around to touch your knees. Repeat this criss cross exercise at least six times.