It is important that your dog has learnt all basic obedience skills by heart and readily complies with your plain commands like come, go, sit, stay etc.
Instinct
Next, you should train your dog to control their instincts around strangers that are already welcomed by you to your house. Here, dogs must sit / stay / lie down on command.
Your dog must thoroughly know what he is protecting. So, give him a run of your property at each session to know its boundaries.
Know Property
Your guard dog should be of a breed having low separation anxiety so that you can safely leave him alone protecting your house.
Low Separation Anxiety
Assessment
Ask someone you know but your dog does not, to approach your property when your dog is guarding it alone. He should immediately bark and let his presence known.
Assessment(cont'd)
Get feedback from your friend and intensify training of your dog if necessary.
Keep testing your dog’s behaviour and reward him with his favourite snack or toy. Let him know that your affectionate pat means that he behaved properly whenever a stranger approached.
Reward
Strangers may use different tactics to divert your dog’s attention. These distractions must already be visualised and tested on your dog so that he does not fall prey while guarding the property.
Diversions
Trust your dog to guard your house properly. Testing him and correction if necessary is a continuous process.