Complex sentence: He caulked it using a mixture of seal blood and flour, and, using wood and nails taken from packing cases and the runners of the sledges, he built a makeshift frame which was then covered with canvas.
Split into simpler sentences: He caulked it using a mixture of seal blood and flour. Using wood and nails taken from packing cases and the runners of the sledges, he built a makeshift frame which was then covered with canvas.