There are several approaches to the choke. But the one that seems to work best for most people is Marcelo Garcia's approach. Their head needs to be facing directly up. Your ribs keep that with contact on one side of their face, and your arm is around their neck, elbow on the floor, as just a "trap" for their head to be inside of. The choke does not come from lateral pressure in this case. It's not a trash compactor type of presure in that way that an arm triangle or a darce is, and it's not a cutting/drawing in type of forearm pressure choke the way that a guillotine is. Once their head is facing up, you are sprawling your weight backwards to open up the space between their chin and their chest, exposing their throat. From there you want to drop your shoulder straight down to the floor, only their throat is in the way. It's still hitting the cartoids. You do not have to squeeze in from the sides to hit the carotids, so the choke will put people to sleep. There's really no other choke with the same pressure, so it takes a while to figure it out. It's one of the few chokes that you can't make up for lack of technique by squeezing. If you don't have a perfect RNC it doesn't matter. You can squeeze and power through it. Not so with the north south coke, as squeezing laterally won't (usually) put them out. So the 3 key concepts are:
1. Your ribs and arm wrapped around their neck just provide the structure to hold them in place to be choked.
2. Sprawling your weight backwards. Even if you think you've sprawled back far enough, you probably haven't. Keep going.
3. Drop your shoulder through their throat, down towards the floor.
The grip doesn't matter so much as long as your choking arm has the elbow on the floor. You can choke with one arm, gable grip, guillotine grip, etc. Whatever feels best.