That wing isn't any kind of flank feather. Those are from the primary flight feathers on the wings. To tie them you need a matched pair of wings, from which you take the same feather from each wing. Teal and mallard wings both produce suitable feathers for this kind of fly wing. (Other birds can also be used, but many don't hold together well enough to form this kind of wing).
Usually slips are taken from the same place on each feather. These are placed together so that the curve away from each other. Then tied onto the shank using the "pinch and loop" technique. They are then stood up using turns of thread. However, this way doesn't always produce good wings, and can be very frustrating. My preference for doing these wings is to use the method I was shown by Donald Downs. In the photos I have used white duck feathers for clarity. I've also tied it a bit close to the eye.
Select your feathers.
Remove the biots from the leading edge of the feather and trim the feathers so you can get at the portion suitable for winging.
Cut slips from each feather by cutting through the stems. (Use heavy duty scissors)
Lay down a bed of thread to tie them onto. Place the slips together, and measure for size, about twice the gap of the hook.
Pinch and loop onto the shank.
Take turns behind the wing to stand it up.
Once the wings are upright take the forward portion of each slip around the bottom of the wings to each side and tie down behind the wing.
Trim out the excess at an angle so you can tie it down smoothly, as you progress down the hook shank
If you want the wings wider apart you can take a couple of turns between them.
Advantages of using this method are; the wing slips stay together better while being handled, and it produces a more robust wing.
In your picture either the slips have not been matched up properly, or both have been taken from the same feather. That is why one slip is longer at the front, and the other longer at the back.
This kind of slip wing on dry flies tends to make the fly top heavy. Often they will fall to one side or the other. Which is why I no longer use them on my flies. I only tie them when a customer orders them. My flies either use a bunch of duck flank feather fibres or poly yarn, Or I use a Wally Wing; which is a whole different story!
This isn't an easy technique to get to grips with. What I have shown you is the best way I have found to do it, short of getting someone else to do it.. If you have any questions about it feel free to ask.