The doorbell devices I suggest are wireless and this will allow you to practice throughout the day at random times whilst you have the buzzer in your pocket out of sight. By the way, I do this with new pups to the house, preventing the habit from forming in the first place!
Disable the existing doorbell, and for the next 2 weeks or so you’ll be without a bell so a sticker on the door letting people know may help.
Your method would be something like this:
Press the new doorbell buzzer in your pocket and say ‘On your bed!’ in a positive bright voice as it chimes. Ignore any barking and direct your dog to the bed area (allowing the dog to drag a lead may assist in directing the dog as you can stand on and then lift this to initially control your dog) and once your dog is sitting nicely on the chosen spot, immediately treat with high value food. This food could be chicken, ham or liver cake as examples. Once quiet and calm, reward your dog and stay calm in your voice so as not to excite. Use of the ‘Stay’ command here will be useful, and combine the command with a flat open hand to enforce the verbal command.
Steadily and constantly feed your dog whilst on the bed. Keep your dog on the bed for 30 seconds to start with and then with each practice aim to extend the bed stay periods gradually each day by a further 10 seconds or less if progress is slower.
Once the time is up on the bed for that session, use a clear ‘OK!’ release command and then walk away putting the food away ready for use next time round. A few of these each day assuming you’re there to practice, will be ideal. Aim for 3-4 short sessions per day.
Later on, once the above is looking good, you could walk your dog to a location near the front door and ask it to sit and stay on a mat (you could clip your dog onto a lead set up there in advance which is fixed to a banister for example) and then go to answer the door having activated the doorbell first. This can be done with your ‘invisible guest’ to begin with, and then to use family members as trials and then arrange to move on to real visitors with the excitement that brings. Really go for it with the ‘invisible guest’ and talk to them at the door as you would a real person whilst keeping a close eye on your dog. Immediately go back and reset the dogs should he move off the location mat.
At the final stage you can fix the buzzer to the front door as your training will be ready to cope with real callers. A second buzzer that you can use inside the home during this settling in or transition phase will allow controlled practice sessions and should further help your efforts.