Focus on the essentials: Introduce your dog to a variety of places, people, dogs and situations they are likely to encounter in their life with you.
Go at your puppy's pace: Observe your puppy on outings or when meeting new people, this will help you establish a list of what your puppy loves and what your puppy doesn't like or might be fearful of. Learn to read your puppy's body language, dogs primary language is body language and you can tell a lot by just watching your dogs reactions to specific situations etc. Observing your puppy will help tremendously in the future when dealing with different situations.
Socialize your puppy individually: Puppies who go everywhere with other dogs or older housemates do not develop the ability to handle situations well on their own. They will likely be fearful to explore the world without their friend.
Utilize every opportunity: Every-time you interact with your dog, they are learning which is why it is so important to use every opportunity you have to socialize your dog properly. The way to create positive associations with new people, places, things or situations is to pair those things with food, or play. When on outings you should always have your socialization tool kit with you 1. Clicker, 2. Favorite toy, 3. Favorite treat (usually this is something soft, moist and smelly!
Be Safe and Sensible: You want to ensure that all experiences your puppy has while socializing are positive ones. This means when out with your puppy you need to be their advocate, be on the look out for dangerous or scary encounters and protect your puppy at all times. Do not let people force themselves on your puppy or force your puppy to experience something that is scary to them!
Interrupt and Prevent: If you see a group of children coming your way and you know your puppy is fearful of them use some of the treats that you have to lure your puppy in another direction and move away. When you are at a distance where your puppy feels safer then let your puppy watch them from a distance while you click/treat.
Get Careless: Avoid places that could effect and or damage your dogs health, mental well being or behavior. Places that are frequented with unfamiliar dogs such as dog parks are a big no no, they can be overwhelming for puppies and can pose health risks such as parvo, kennel cough, intestinal parasites and even physical injuries.
Befriend every dog you meet: Do not allow your puppy to meet any and all dogs, not all dogs are friendly or healthy.
Force the issue: Never force your puppy to interact with something that is scary, this can cause more fearfulness and even aggression. Instead move your puppy to a safe distance where they are comfortable and willing to accept treats. At that distance you can work on building positive associations and gradually move closer to the scary object, person or other animal as your puppy feels more comfortable.
Punish or correct your puppy: Punishing your puppy for fearful or aggressive behavior will only make things worse. Dogs learn though associations so if your dog is already scared of something and you correct or punish them around it, they will become even more fearful. Punishments should be avoided at all cost, dogs are amoral and do not understand right from wrong.
Overwhelm your puppy: When meeting people keep it simple at first, only allow one person at a time to greet your puppy. If you let a whole crowd of people greet them at once they likely will be overwhelmed, which can lead to a fear of people. The same rule would apply to meeting other dogs, one at a time! Dog parks are overwhelming even for adult dogs, could you imagine a swarm of dogs you've never met charging you as you walk through a door...Scary right?