Go to the Scrapyard, find the Dog (called Dogmeat) and get him to join him.

Now go on a few missions and make sure he dies in them. Then load your last saved game and give him a stimpack. Repeat this.

Now load the last save and go to Megaton. Leave him in your house until he leaves and then a note will appear saying that Dogmeat is at Vault 101.

Go get him and shoot him once with any pistol. Then heal him and the cheat will be activated - he can't die!