Using your left hand to open your opponents collar slightly, use your right hand to grab the collar with your fingers on the inside (thumb outside). Try to get a very deep grip otherwise the choke may not work.
Take your left hand underneath your right, and grab the collar on the otherside exactly the same way (fingers inside, thumb outside). Make sure you get a deep grip with this hand aswell, ideally your fingers should be nearly touching at the back of your opponents neck.
Now pull your opponent in and push your elbows out. Do not try to choke your opponent with brute strength, simply pushing your elbows out will apply the choke much quicker.
This variation is applied the same way but with your thumbs on the inside of your opponents collar. Remember to get a deep grip with your first hand.
Take your left hand underneath your right and get another deep grip.
Again just pull your opponent in and push your elbows out. If the choke does not come on it means that you probably didn't get a deep grip with both hands.