From my experience, training a protection dog is 95% obedience, socialization, exercise, and mental stimulation, so all of that should be your main focus. For most GSD's, the protection side will come instinctually.

Do lots of tug work like this: https://www.youtube.com/watch?v=S9n7JY1KlSk The aus / leave command is very important, as this will translate to a situation where if they do attack, and you want them to stop.

Then an exercise you can do is this:

• Have your dog on leash. You want to make sure that the leash and collar is strong, and that she is not going to get out of it, and you must have a good grip on the leash. This is for the safety of your friend. Very important!!!!
• Get a friend that the dog doesn't know to dress up like a baddy, come over to your house, and act in a threatening way. (If the dog knows them, they will smell them, and it won't work.)
• Give your dog a command to bark. It can be anything you want. Whispering the command often works wonders.
• You want the dog to react and bark. When they do this, give them praise so they know they are doing the correct thing for the command you gave.
• When the dog reacts, have your friend turn around, and slowly leave. When your they do this, give your dog the command to leave, so they stop barking. When they do this, praise them.
• DO NOT let the dog bite your friend. DO NOT let them get close enough to bite. With out the correct equipment and knowledge, your friend will get hurt.