Start by parting a circle around the head, leaving out a headband under the circle.
This is kind of hard to tell in the picture, but you start french braiding from the bottom-middle. Grab your three strands to start with from the hair that is out of the pony. Then as you braid, you bring small pieces from the pony tail from the one side & small pieces from the hair left out from the other side. You can either do inside out french braid or a regular french braid. I actually started with a regular french braid & didn't get very far, I felt it was hard to position my hands, so I stopped very quickly & redid with the inside-out french braid & it was very fast & much easier for my hands to hold the hair. But whatever works best for you - try it!
One you've gone all the way around the head, finish braiding to the end of the hair.