Probably what the Mongols used to do...
They would take the prisoner and tie them to the ground by their wrists and ankles in the middle of the desert, giving them water, but every day, they would go and cut off one of their digits on each limb. First the fingers and toes over five days, then the hands and feet, then the forearms and forelegs, then the legs, then they would cut your body and remove non-vital organs until you died. They would burn your cut-off-parts to prevent you from bleeding to death, and would let crows and vultures pick at you after all you had left was your vital organs. That was for criminals, though. If you were a king or something, they'd just throw heavy rugs on you until you suffocated.
