I just copied this from a website it that's ok :) Remember: ONLY do this in LIFE OR DEATH situations, or if you are in a clear and present danger.
1. Wear a protective glove that covers not just your hand, but also your entire arm. This is a necessary safety precaution to prevent injury.
2. Sit your dog down. If you have not taught it simple commands like come, sit, stop, run, and stand, then you must teach those first before teaching the “attack” command.
3. After sitting the dog down, tap it in the face with the glove on your arm. This is a way of irritating the dog and trying its patience. Continue doing this until your dog attacks the glove in anger. (Do you now understand why you must wear an arm-long glove?)
4. As soon as the dog attacks the glove, say the word “attack” loudly. You are doing this because you want to make your dog understand what the word “attack” really means.
5. Praise the dog verbally or show it that you are pleased. Though it may not be necessary, reward your dog by giving it some cookies or other treats. But if your dog is often motivated by food and learns faster with treats, then you must offer it some.
6. Repeat steps 3 – 5 until you are sure your dog has understood what the word “attack” means. You will know this if your dog responds quickly to the “attack” command by attacking your gloved arm immediately it hears the command.
7. Stand at a short distance away from your dog, and give the “attack” command. See how it responds. If it doesn’t, then repeat steps 3 – 5 many times over. But if it attacks, this shows it’s familiar with the command.
8. Take a break to reward the dog once again for understanding your command.
9. Repeat steps 7 and 8 for five to seven more times. This is to ensure that your dog has really mastered the command.
10. Now, it’s time to show the dog how to attack an intruder (or else, every time you say “attack”, it will go after your arm). Get a very big doll or make a caricature human and place it at a distance. Then point towards the “fake” intruder and shout “attack”. Although your dog may attempt coming towards you at first, it will go in the pointed direction once it sees your finger.
11. Reward your dog with treats or in some other way.
Repeat step 10, placing your fake intruder at various points and pointing at it each time you command your dog. Repeat this until you are convinced that your dog has mastered the command and would no longer come after your arm.
12. Get someone to act like an intruder (use someone whose face is not familiar to the dog). Tell the person to stand by the door entrance and get ready to shut the door in case the dog ignores the “stop” command.
13. Point towards the “intruder” and command your dog to attack. If it runs towards the intruder, then it has gotten your message.
14. Repeat step 13, but while the dog is going for the intruder, shout the word “stop” to see if it will obey. If you’ve familiarized it with the “stop” command, it should stop immediately. This step is very necessary because it teaches your dog that sometimes you may want to stop if from attacking an intruder after initially giving the command.
As a final note, I believe that with these steps, you would successfully transform your dog from a mere pet into a selfless guard dog that attacks only when you want it to.