This is an abdominal exercise from Pilates, not traditional yoga.
Preparation: Lie on your back, anchor your lower back to the mat, and bring one knee at a time toward your chest. Then, with your legs flat, put your fingertips behind your head and lift your head and shoulders off the mat. Staying up, and not bringing your elbows forward, lift one armpit higher. Return to center, but do not let your shoulders drop back to the mat. Still up, lift the other armpit. Check to see that your elbows are staying back. Keep your ears parallel to your shoulders. Maintain a fist-size space between your chin and your chest. When you feel that you can lift one shoulder and then the other without falling back to the mat or letting your head flop, proceed to the criss-cross:
Alternating legs, bring one knee towards your face while the other points straight out, parallel to the floor but not touching the floor. Each time a knee comes toward your face, lift the opposite armpit toward it. Check that you are not flopping back to the floor, and not bringing your elbows forward.
At your own pace, do three sets of eight repetitions, and keep breathing. Do more if you can.