Aperture corners can be found by doing the aah-ooh as demonstrated. It is important the corners of the lips have tension. This is your anchor and I find that keeping a stable embouchure firm lip corners are essential, but this is not your aperture corners. These firm lip corners are NOT created by smiling or frowning lips but are located on both sides although there usually is a slight frown when engaged.

Let me stress the corner lip tension are NOT aperture corners but are lip corners. I read about a guy who used toothpicks in his lip corners to get these establish. As with anything, too much tension can be focused here. Getting the right balance without excessive tension is important.