Allow your puppy to explore, safely of course, and encourage them to approach scary, but safe, objects.
When your puppy tries to hide, run away, or barks at something new and foreign, check that it’s safe to approach, and ask them, “what is it?” Encourage them to cautiously approach.
Peak their curiosity. Approach with them and touch or slowly move the object of their fear.
For example, one of our puppies-in-training, Tommy, saw my toddler’s toy lawnmower one day sitting in the middle of the yard. He did the typical “Bark! Bark!”, run-and-hide-behind-mom maneuver. So we took the opportunity to show him how to overcome his fears. “What is it?” We asked. He barked, but stayed back. We slowly walked forward and said “What is it” again. He edged forward and sniffed. Once we touched it, he quickly backed away, and we started back at the beginning until eventually he was able to touch it himself and see that it was just a toy, an inanimate object. Then we slowly moved it to a new spot in the yard. He went back to hiding and barking, but he overcame his fear much more quickly this time and was fine with the toy from then on.
Below is a video of Scout, a 3 month old Scottish Terrier. While here for puppy daycare, he noticed a very scary object in our daughter's room. Watch how we handled this situation and apply what you learned.
The moral of the story? Be calm and patient, but also encourage their curiosity and show them it’s not something to fear.