The first thing you want to try and do is once you've grabbed your foot, pull up. Literally think about pulling your foot upwards as high as you can. Once that's easy, try leaning forward a little bit while doing the same thing (it makes your scorp higher). Then try reaching further down on your leg and doing the same process. It takes time, but it's worth it!
I do think the best way to get a good scorpion is to practise pulling it over and over, but there are stretches you can do to help. Try sitting in splits and reaching back to grab your scorpion leg. Try door-frame stretches (see Gabi Butler's stretching video on YouTube). Try rocking back into rainbow bridges. And, most importantly, ALWAYS stretch both legs.