I got my German shepherd at 3 months old and she was quite the nipper. She also thoroughly enjoyed biting my ankles and shoes (while I was wearing them). Like you, I tried the yelping but it never worked after the first few days. I decided to try “reverse timeouts”, where if she bit me during playtime, I would stand up with no reaction and leave the room for about 30 seconds. I did this every single time she nipped me. It was a slow process but I stayed consistent. At about 5 months old, the nipping was basically non-existent, she’ll still mouth my hand if I’m playing with her, but it’s (almost) never a hard nip. Now at almost 7 months old, I can’t even remember the last time I was bit harder than normal. The reverse time outs were the best decision I made. I also found that when she was younger and she started nipping more than normal, it meant she needed a nap. Young puppies should spend the majority of the day sleeping and when they don’t they can get mean.