You begin in closed guard. You dig your left hand to their left side collar and pull them down to break their posture. You dig your right hand to their left side low lapel. If you can dig it and pull it out with your guard closed, that is good. If not, you pressure down with the right hand while you break your feet, grab and pull with your right hand, and recross your feet. You angle to your right side a bit and loop the loose lapel over top of your shin. Keep them close with your collar grip, let go of the lapel grip with your right hand, dig your right hand under their left knee and grab the lapel.
Now you are in a great position. The lapel is connecting their knee to their neck or their ass to their neck. Your grip on the lapel is strong because of the physical profile of the lapel – you can hold it very well. They cannot posture up because they will be constricted and the distance between their neck and their knee or ass can only be as far as your grip allows. Be sure your hand is dug far enough that the blade of your forearm rests under their knee or leg. That way, when they try to posture and your grip gets tested, the pressure is on the blade of your forearm and not on your hand.
Now that they cannot posture well with their hips and back, they will place their right hand on your body to push themselves up. When they do, let go of the collar grip with your left hand, and grab their right side sleeve and pull archer style to break their post.
Drop your left foot to the mat and use to rotate your head to your right side and swing your whole body 90 degrees. Now your left leg is free like a pendulum if need be, and your right leg is pressured straight into their body under their arm/into their ribs.
From here, if they do nothing, they will be swept to mount. However, most people will post or pressure or freak out in some way to keep their base and not get swept. When that happens, you place your left leg over their face and hip in for an armlock.
Professor also showed that there are triangle choke and omo plata opportunities, and that you can instead throw your left leg over top to their left side for a reverse sweep to side control.