Well I am no expert.... but I have just started teaching mine after I realized that the way they stand can potentially place them low in a class....
Stand infront of them...
walk them towards you and when you want them to stop (holding the lead in the left hand) pull the lead sorta up into the air,this means stop.
Then check their back legs,if they are standing sqaure in the back well and good if not push them BACKWARDS never bring them foreward to fix it,take as many steps back as you need
Next once you have them square bring you lead back up into the air then (I have horses not ponys so they stand different) I take a bit of food (mine are food scabs and will do anything for a biomare cube) I ask them to stretch out using the bit of food,you will find the will probably move their back legs and try to walk foreward,stop them and back them up again till their back legs are square and try again.
It will take days for them to work out that you want them to keep their backlegs anchored to the spot,just keep asking them to bring their front legs foreward,the moment the back leg moves,nope not good enough back them up a step!
Eventaully they will have a light bulb moment and bring their front feet foreward and sqaure a touch,reward them as soon as they do it,lots of praise for them!
then once they have that worked out I ask them to stretch a little more each day.
Clear as mud,yeah? i will try and get some batterys for my camera and take vid of it.... but it will be shoddy because well it will be a one man production LOL!
Basically with my colt all I have to do is hold the lead in the air now he plants his back feet and stretches out by himself,infact in the paddock yesterday I asked him to stand and he done it at liberty,but I am really getting him to stretch out where as the ponys dont so much do it I think