After the correction and praise there is nothing wrong with trying to play with a ball and string. This is another way of teaching the pup what is an appropriate prey item and what is not appropriate. You will know that you are going in the right direction when the pup is about 4 months old and he starts carrying his tennis ball around or his rolled up tug towel. This is his way of showing that he knows what his prey item is. When you see this, take a minute to toss the ball 2 or 3 times for him.
New handlers seem to think that training needs to be regimented.
I feel its much wiser to work the training into the dogs normal life. A few minutes here and a few minutes there when the moment presents itself is much more effective way in working with puppies.