This choke can be used when your opponent has atleast one hand on your chest. In this case we will use the opponents right arm.
Bring your left hand inside your opponents arms.
Push your left arm out to the side and upwards to slide your opponents hand off your chest. Now wrap your left arm round the outside of your opponents arm.
Use your right hand to open up the left side of your opponents collar and grab it with your left hand (fingers inside, thumb outside). This locks up your opponents left arm.
Slide your right hand in the other side of your opponents collar with your thumb on the inside (fingers outside).
Now push your right elbow up to the ceiling to apply the choke.