I also extend the hind leg to the back then pull it forward. I usually do the front legs while I'm at it too.
Extend their back legs behind their body. I hook my arm under my guys hock and slowly encourage him to bring the leg back, don't force it but support the weight, can also scratch on one side of the tail and watch their opposite stifle until it contracts or do tail pulls to either side. Try Google-ing YouTube videos for horse stretches.
Keeping your horse round while walking up and down hills ( they don't have to be very steep.)