Your technique is good so it's a matter of perfecting. When throwing in from standing position make sure to make a full swing with your left leg. In the video you go to the handspin/handglide position a bit too early. You can also make the entire swing bigger by first putting down your left hand and shortly after that your right hand.
Now to the windmill itself.. you're swinging your legs which is great, now try to start twisting the hips too. Allow me to explain (and I'll use your body as an example
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When you're on your back about to roll to your front make sure you twist the hips in the direction you're going. The picture below shows you what happens when you forget that: the legs are behind and you have to bend your knee to pull your leg through. It looks a bit as if you're legs are in a front split (left leg front, right leg back) while your legs should be more in a middle-split (both legs to the side).
I did a little photoshopping in an attempt to show you what happens when you twist your hips when you roll onto your arm:
Not entirely realistic but I hope this makes it clear
What you can see it that your legs are not as far "behind" and that you can keep your legs straight. In clockwise windmills you have to twist your hips to the left. The yellow line gives an idea of how the hips are positioned.
This will also allow you to swing your left leg towards your face when you roll off your arm, giving you a better swing and thus more speed. After that comes your right leg and then you can twist your entire upper body for even more speed. When your windmills start to go fast it becomes more a matter of upper- and lower body twisting rather than swinging either your left of your right leg.
Windmills are hard, I'm just getting to the point where I'm starting to twist instead of separate leg swings. Just keep at it and you'll get there!
