We taught it by having horse plant back feet and moving front feet one at time, by bumping coronet band lightly with toe of boot. Do this slowly, moving from one side to other, at same time making sure the back feet stay in place.
This is one reason I always taught a "be still" command too.
At same time? You need to be using lead shank to raise head so horse is stretched out in neck and looking "in the spotlight".
Give command to Park Out, split second after you raise head with shank, so when you cue horse with head being raised, those feet park out like supposed to.
And when you are up? You can set back feet and then "toe" the elbow to get horse to move one front foot at time forwards.
Originally, this was taught to get horse lower for mounting from ground, apparently?
Horses make me a better person.