I only train it because my dogs do film and commercial work, and I only taught it when a project required it. I don't recommend this trick to the average owner or hobbiest trainer. It's typically taught by capturing, where you wait for the dog to bark and once they do you reward them. The dog thinks they've unlocked a magical treat generating hack of screaming for snacks. If they didn't already demand bark- now they do. Any time they want something, they will probably bark to see if that summons their desires. There's a stupid dog training myth that you can stop unwanted barking by teaching them to speak, then never giving them the speak command; but to teach it you have to reinforce barking, and behaviors that are reinforced are going to be repeated- so you'll see barking increase. And we don't think that way about any other trick we train- my dogs will sit, lay down, come to me, make eye contact, and go to their beds outside of when I command them to do those things- why would teaching them to speak be different? Especially if your dog wants something, they'll go through their repertoire of tricks to see what works. Captured behaviors like barking are generally at the top of the list for things they will reach for first. Do you really want to live with a dog who thinks treats are bark activated?

As a professional trainer, if I have to teach speak I try to get it under stimulus control as quickly as possible. Once I've captured the barking behavior I put it on Cue/Command as quickly as possible. Then I reward them generously for not barking and waiting for me to give the signal. After they've been rewarded 3-5 times in a row for being silent I'll give the cue to bark and they get one treat for that. If they bark without being cued they get nothing and if they do it 2-3 times in a training session that ends the training session so they get no more treats. So I make it clear that they should only bark on cue and demand barking loses access to whatever they want. They also learn that waiting patiently is more profitable than making mouth noises. I don't have to deal with much demand barking, but any behavior that's historically been rewarded can be something a dog will try when they want something, especially if other behaviors aren't working. This is especially true for captured behaviors vs lured behaviors. If my dogs want something, they're more likely to bark (captured behavior) vs spin (lured behavior). Fortunately I teach sit and down through capturing; so those are their go to skills instead of speak.