Stand by your horse's side and ask him to reach his head around with a treat. You could also do this on his back w/o the treat just using reins
Never stretch cold muscles. Just like us warm up the muscles with hand walking , light in hand lateral work before picking up the limbs and stretching. Stretching and massaging the muscles after the work out will benefit the next days work.
All my stretches are from EventionTV . They can explain it better than me but I pick up my horses front legs and pull gently out towards his nose. Don't pull to hard , just until you feel him lean back a bit . And if he struggles go a bit slower