You would think the police would have a preferred method for teaching something like this...
I don't have much experience with this sort of stuff but I did a little bit last year with a military bloke and a couple of my dogs...
We started out in a room with 4 doors off it, all the doors closed, and the decoy behind the first one with a sleeve... We made sure he was right behind the door and giving the dog the best chance to get the scent... I would then bring the dog into the room and lead him to the first door and when the dog showed interest in the door I would quietly encourage him and when he sniffed at it I would say "good boy!" in a loud voice and the decoy would open the door and give the dog a bite...
We just worked on the first door for quick sessions for about a week and the dog began to automatically run up to the door, have a good sniff, and then we got a bark alert through frustration... Once the dog was solid on run to door, sniff and bark, we left it a few days so the scent would dissipate a bit and then put the decoy behind the second door...
The dog of course ran to the first door and sniffed and before he could start barking I called him to the second door (dog still on leash), and told him to sniff there... He did and when he got the fresh scent of the decoy he gave a bark and got his reward... We worked the second door until the dog started to run past the first one to get to the second and then we put the decoy back behind the first door and let the dog run past to realise he'd missed his reward and he came back to the first door...
We did a few first doors, then second door, then finished on the first one... Then once the dog was reliably checking the first door on his way to the second when we mixed it up we added in the third... Same thing as before, dog would check first, check second, and then I would call him along to the third...
You have to make sure you teach the dog the pattern and that he must check every door as there is a chance of reward from every one, and we would always finish the session with a first door hide, so the dog never got in the habit of running past the first door as so many do because they are keen for action which always seems to occur further into the building...
Someone with more experience may be able to expand on this and add more steps but that's my limited experience with door-to-door building searches...