How does the Ramzi theory work?

Carly • 3 little girls! Riverlynn 6/18/2013 & Rynnleigh 8/13/2017 & Rosalin 11/6/2018 and 1 little boy due 5/8/2021!
I've had an emergency u/s so I didn't get any print outs and won't until the 23rd. But we did get to see baby...anyways could someone tell me how the Ramzi theory works..if I were to do it based on u/s?