He planted a field next to someone else's roundup ready crop. He then collected the seeds from the whole field. Next year he planted the seeds collected, waiting for the crop to start growing and sprayed roundup, which killed all seeds not roundup ready. Then the new crop went to seed and since all the parents were roundup ready, the seeds also were roundup ready (I'm not sure if roundup ready is a recessive or dominate trait, which will influence which % is roundup ready using basic genetics). Those final seeds were collected and were enough to have a whole field that was roundup ready next year.