Use your left hand to open your attackers collar and slide your right hand in with your fingers on the inside (thumb outside).
Make sure you get a deep grip with your right hand, otherwise the choke may not work.
Take your left hand under your right arm and use the same grip as before. It is important that you get a very deep grip with both hands for this choke. Ideally your fingers should be touching at the back of your attackers neck.
Now twist your wrists so that your thumbs are next to your attackers neck.
Lean over your attacker to one side and put your head on the ground. Now just push your elbows out to the side to apply the choke. Do not be tempted to use brute strength to apply this choke, the elbow movement is all that's needed.
This technique is the thrusting choke applied from the mount.
Grab both sides of your attackers collar quite low down with your fingers on the inside (thumbs outside). Your thumbs should be facing you.
Drive your right fist into your attackers neck just to the left of their windpipe. Pull down with your left hand to remove any slack from the collar.
This is the bar choke applied from the mount.
Use your left hand to open your attackers collar and grab it with your right hand (thumb inside, fingers outside).
Your right fist should be touching the floor.
Grab the other side of your attackers collar with your left hand and pull down to remove any slack. Now push your right elbow down to the floor to the right of your attacker.