In this short video, Greg and I demo the rear naked choke from a standing position.
To enter, you've got to loosen up your partner somehow. Here, I've done a groin shot - but the possibilities are endless. Whatever you do, it's got to give you enough time to get to their back. Once your there, slip one arm around your partner's neck. Put the front of their neck right in the crook of that elbow (in the video, Greg is awfully generous in the placement of his neck). Grab the bicep of the other arm with the choking hand. Now take your free hand and grab the back of the head. As with all submission moves, less space for them to squirm = better.
Now slowly (because you like your partner), squeeze your arms while you press forward on the back of their head and pull back on their throat. If they are taller than you then kick them in the back of the knee to bring them down.
The escape that Greg demonstrates is one of many possibilities. As soon as you feel that arm slide around your neck, grab at the elbow pit with your close hand and pull! Create some space so you can breathe. From there, turn towards the bicep and keep turning. Climb the arm like a rope. This escape is by no means foolproof. Every move has a counter to infinity. Play (safely, of course) and see for yourself.