How did the instructor make Douglas a good swimmer?

Bit by bit, the instructor turned Douglas into a swimmer. Initially, he put a belt around Douglas and the attached rope went through a pulley that ran on an overhead cable. He held on to the rope and went back and forth across the pool hour after hour, day after day till he began to relax in water. Next, the instructor taught Douglas to put his face under water and exhale and to raise his nose and inhale. They practised this until Douglas stopped panicking on putting his head under water. Then Douglas learnt to kick in the water. At first, he was terror-stricken but continuous practise with his instructor, made Douglas lose his fear of water and emerge as a confident swimmer.