Bend a half curl in the wrap wire an shown in the photo displaying the front wrap. Use this technique for the back and wrap it first.
Wrap the wire so that it pulls tight and straight across the back. This will push the stones out the front. Once tied off and trimmed, flip the piece over and using your thumbs push the centerpiece down into the frame. This will "knuckle" the back strap and make a nice bed for the piece to lay in. If you prefer, you can install the front single wire wrap now. Or, you can wait until last if you wish. Either way, the front wire, after installed, is pushed downward with your thumbs from the top.
Work the center of the wire so that it slopes gently into a curved "label" effect as if on a bottle.