On the one hand, it would be great if your opponent would simply make the fundamental mistake of turning away from you, essentially halfway giving you their back, so that you can easily transition to technical mount, and then on to S-mount. However, let's start by assuming that our partner isn't quite so gullible.

Instead, as your partner begins with their elbows in super tight, start with a low mount (I prefer having my ankles crossed beneath their hips, just like with the reverse armbar set up, but that's up to you). From here, scoop under their head with your left arm, palming the side of their head as shown in the video. Next, pull their head up, and then to the side (to your left side). Now creep your left knee up underneath their shoulder, and pinch your knee in tight. Notice that your partner will make "accordion arms" here, with their elbows, flared out. Lean all the way to your left so that your right leg is free to kick straight out, and then underneath your partner's armpit. The ultimate place you want to be is with your partner's head on your knee, like a pillow, and your legs perpendicular to your partner.