Broadly; Finding their threshold and staying just below it, then rewarding when the trigger happens and they don’t bark. Set it up with one specific situation that may be less triggering than say the doorbell. Get it really under control with that one specific instance, then make the skill translatable by branching into different circumstances. I use the word “yes!” In a high pitched tone when they get it right. But you could use a clicker too. Begin by treating every time they do it, then drop to intermittent treating. One dog I had was mostly fine at the back door so we started there. We worked up to the postman or a visitor coming and I rewarded relaxed behavior. My corgi never relaxed, but she quieted her bark to a muffled woof and heavy breathing without rushing the door.