The pilot reported that before takeoff, he instructed passengers to position themselves sideways for landing, with a shoulder facing the direction of travel. At the conclusion of the balloon flight, the pilot anticipated a bumpy landing due to an uneven landing area with knee-high bushes and winds of approximately 14 mph. He directed the passengers to get into the landing position that he had briefed. During landing, the balloon slid approximately twenty-five feet and rolled over before coming to a stop, which resulted in serious injury to one of the passengers. The pilot reported there were no preaccident mechanical failures or malfunctions that would have precluded normal operations. He further reported that he was not aware of the manufacturer’s landing procedure, which advises passengers to turn backwards so their back faces the direction of travel and to secure themselves using the basket handle or tank collar.