Like this:
or this?
Is actually quite easy. I showed Morgans for years and this is about the ideal for stretching IMO. Some horses get to like posing this way and will over do the pose. I start with a little--teach them to stand square first, then gradually move the front feet forward until they're where you want them. Then of course you want them to lean forward so that they're right up over their front feet, with their croup flattened. Nothing looks worse that stretching a horse & then having him stand there in sawhorse stance--leg at each corner, butt up in the air, back hollow. It's helpful to start out posing the horse on an incline, front end higher--the incline will encourage the horse to stand over its front feet & it kind of conditions the horse to stand correctly once you start posing him on flat ground.
You can also coax the horse to "lean" forward by baiting him--I'll ask him to rock just a little bit forward, hold it for an instant, then tell him to come, step him out of the pose & reward him. Gradually lengthen the time you ask him to hold the pose. As you ask him to lean forward over his front legs, you can encourage him to keep his feet in place by blocking his chest with your hand. With the Morgans you cannot show with "bait" at all, but our horses learned that when I posed them & put my hands up, they were to reach forward & then hold the position. I never showed with a tailer, so had no one else there to help alert the horse in the ring.
Showing Minis isn't so different. I do have to watch not to over-stretch them for Miniature classes, but I do like to show them "parked out" just slightly. To me it's just a more polished look, the difference between showing a Mini Morgan as opposed to a Mini QH!