Remember stretching is only half of the equation when it comes to flying. You can be the most flexible person in the world, but without the muscular strength to actively move through each body position, while remaining stable, flexibility wouldn't do you a bit of good.
I suggest using resistance bands/tubing to do exercises to strengthen the muscles of the leg, and doing stability exercises to help strengthen the muscles of the core, in addition to stretching multiple times per day, at a frequency of 3-5 days per week. Using a foam/massage roller to loosen up any tightness or knots in muscles (trigger points) before stretching allows the muscles to get to their optimal length, and will help you see improvements in flexibility quicker.
Most importantly, remember that you should stretch until you reach your comfort limit, hold that stretch for 15-20 seconds, then try to stretch a little deeper. Repeat this 3-5 times. DO NOT stretch too much, too quickly. Stretchto the point of discomfort, NEVER to the point of pain! Pain will cause your muscles to tense up unconsciously, as a protective reflex, which could cause small muscle fiber tears, which would set you back in flexibility.
I personally have worked along side Gabi Butler (and her family) at a stunting/stretching clinic, and can vouch for what I have seen of her flexibility/conditioning routine! I was very impressed by how much she knew, for such a young girl!
Anyways, I hope this helps. Best of luck with flying!
-Matt Faherty