The best way to teach a dog to be quiet on command is to teach them to speak on command. That’s the only way you can practice telling them to be quiet.
Putting a behavior on command requires repetition. The only way to practice shushing is to practice barking. Use an accomplice and an appropriate stimuli to train the dog to bark on command.
It’s easy to lure a dog to stop barking: simply present a treat for them to sniff. They can’t bark and sniff at the same time. You needn’t always reward them with the treat, but you should praise them as soon as they stop barking.
An older dog is no excuse. If you don’t want your dog to bark at the door, teach them to “Shush” on command. Challenge extended!
Some dogs get extremely worked up when visitors ring the doorbell, or when dogs walk by the house. Some spaniels and terriers bark at the drop of a hat. And our good friend Larry Labrador will bark whenever a leaf falls from a tree three blocks away. Barking is as characteristically doggy as wagging a tail or burying a bone. It would be inane and inhumane to try to stop your dog from barking altogether: "You’ll never bark in this town again!" After all, some barking is extremely useful. My dogs are much more efficient than the doorbell and much more convincing than a burglar alarm. The goal then, is to teach dogs normally to be calm and quiet but to sound the alarm when intruders enter your property.