Clark Gracie shows some tips for the rear naked choke on Stephen Kesting.
Start with head and arm position with one of your arms over their shoulder, and your other arm under their other shoulder. Use the back of your head to help you keep your position, tucking their head into the area between your shoulder and head.
Walk your hand (on the choking arm) over their shoulder and grasp their shoulder as close to their neck as possible. The other hand releases from your opponent’s chest to slide behind their head, with the back of the hand against the back of their head. Then, the hand of your choking arm grasps your other bicep. You do not want to bring your arm forward, risking that your opponent will grab it.
Your elbow should be in line with your opponent’s chin. This makes it harder for your opponent to defend the choke because it removes space that your opponent could take advantage of to pull your arm away from their neck.
Put your head tight to your opponent’s head so that your opponent cannot pull your other arm over their head to defend.
The blade of your wrist behind the head should be pushing forward during the choke.
Squeeze your elbows together and flex to finish the choke.