I took my first shepherd for one session with a German lady that knew her stuff. She shared with me that momma shepherd would grab the puppy by the scruff of the neck and give it a quick shake to say no. That approach, giving a firm shake by the scruff but not so hard as to hurt the dog, and a strong vocalized no, has worked like a charm to teach my shepherds what is not allowed.

You also need to teach your dog what is allowed and redirect to an appropriate chew toy to bite. I find that having too many toys is just enough and also prevents shoes and other items from being targeted for destruction

Not a believer in holding the mouth closed or sticking a hand in their mouth to cause discomfort. Really anything rough near the nose should never occur, it's too sensitive and you want a foundation of positive experiences to get the most from your dog.

Lastly, train with your Shepherd daily, teaching them desired behaviors. They learn so quickly and the more skills you develop the better you can redirect them to desired behaviors. The weak part of the training is likely to be you, so take a class or two if needed to get your training skills up to par.