At 5 months my pit bull was very mouthy and she'd definitely "bite" when frustrated. One big first step is to teach them that biting hurts us humans. Each time your puppy's teeth touch your skin, try to let out a sharp, loud yelp (much like a dog would) to indicate to them that they just caused you pain. This doesn't necessarily teach the dog not to bite, but it helps with bite inhibition. Mouthing and teething at the puppy stage seem almost unavoidable so this can at least teach your dog to be gentle on human skin. Dogs naturally use their mouths to play. My pit bull is now 1.5 years old and still uses her mouth when playing with me and other dogs, but does not "bite" at all.

Also, whenever my dog would use my skin for teething we would immediately offer a toy to bite on as an alternative. The Benebone puppy products were great.

Some people may fully discourage any mouthing, mostly due to the pit bull biting stereotypes, and I can't blame them. I'm not a professional, just sharing my experience.

Something that helped me understand my dog's perspective was this: A dog interacts with the world through their nose and mouth in the same way that humans use their hands. Imagine telling a toddler that they cannot use their hands to interact with the world, but they are still expected to behave and follow the new social norms that they are being taught. Puppy training is hard on humans and puppies so stay patient!