Speak on Command

Does anyone know how tho teach a dog to speak on command? Harley is not very vocal, and I would like a heads-up when someone is at my house. People have walked right in my back door, and Harley just gives then a welcoming wag of her tail. I've read dog's have an uncanny ability to know when everything is alright, and when it's not, but I'm hearing impaired, and that was one of my reasons for wanting a dog. For instance, I can't hear my smoke detector, so maybe after she's learned to ''speak" I can train her to alert me to that. I love that my dog is so friendly and has excellent manners, but a heads-up when company's arrived would be cool, too. I guess the advantage of having to train her to do it is that I can only have her bark once or twice. So, how do I do it? I read somewhere that we should get in "training mode", put Harley on my left, tell her "speak" (or whatever) and then start barking myself like mad. This sounds very silly to me. Is this actually how you do it?