Once you begin training your Pom, it should be done every day until the training is complete. This will begin by having someone arrive at your home. Whoever arrives should have dog treats in their pocket. When you know that your helper is to enter, have your Pomeranian sitting down beside you, with his leash on. Open the door and teach your dog that all is fine. This is done with calm words and slow actions. Have your dog see that you greet your visitors without any tension.
Have your helper give your Pomeranian a small treat, making sure to save some for later. The goal of this training is to reward your dog with good behavior and to socially isolate your dog for bad behavior. When your Pom interacts well, even for just a minute, you must behave as if your dog just did the most wonderful thing in the world. Give treats, pats, happy words and praise.
Any time that your Pom shows aggression, bring your dog into another room that is not occupied where he has a view of you but cannot reach you (placed in a playpen for example). You must then implement social isolation for 5 minutes. This means that you must ignore your dog completely. Do not say "bad dog", do not say his name, do not talk to him. Simply stand and allow him to see that life is certainly not fun when he does not behave. After the 5 minutes, attempt to have your dog interact with your helper again.
Each action, good or bad, must be acted upon with either praise or isolation. Have the 1st visit last for about 10 minutes. Have different helpers come to visit on different days. Increase the visits by 5 minutes per day until the maximum time of 30 minutes. By following this training method word for word, your dog should be very used to "strangers" and much better behaved within 2 weeks or so.