They scraped off thin shavings from the inner side, but they were careful not to spoil the skin by cutting through it.
One side of scraper
Opposite side of scraper
When they had a smooth surface, they rubbed it with fat so as to make it soft.
Sometimes they chewed the hard parts to make them soft.
When the skin was smooth and soft, they dried it in the sun.
Then they took it into the cave.