From mount, open the collar on the left side with the left hand. Palm down, four fingers in. Pull the gi open, stick the right hand in, palm up, thumb outside, and slide it up as deep behind the neck as possible. Ideally getting the right thumb to near the middle back of the neck. Keep the right palm facing up and lean the forearm across the neck using the top bone-y part of the forearm, and then push up the chin. Sneak the left hand under the right and get as deep a grip as possible, ideally getting the left thumb to near the middle back of the neck. Keep the palms up, top of the wrists and bone-y forearm pointed into the neck. Keep the elbows together and pull down, rotating wrists down in a similar fashion to a drum hit with a drumstick.
Once the grips are set they can be held through a bride and roll. Establish a closed guard with the legs, getting them high without letting the grips go, then apply the same way using the legs to pull them up. The choke can also be set up from closed guard.