Don't expect too much too soon. It takes developing muscles in a new way to hold that stance. Even for horses that have the conformation to do it well.
Have fun and send pics when you get yours going on this.
It doesn't take any time at all for them to learn this, and any horse can do it for that matter. And doesn't take developing muscles either for it.
This should take no more than couple of times to teach, and have horse doing it well as feet are moved fore and back.
And moving front feet forwards while the back are still is best way to teach it. Backing a horse into it is counterproductive, and I've never seen anyone teach it that way for that matter. Somewhere some may? But not in any training barn I've been in, nor done by anyone I know. We all plant backs, tap them forwards on the front.
And dragging it out by eventually teaching a voice command? Is also counterproductive. Horses can learn something like this all together, no need to make it harder than it is.
One of easiest things to teach actually.
Pretty similar to what they do when urinating in stance for that matter.