In my experience, birds typically learn different words and phrases simply by listening to their owners say them over and over again. I've not ever in my life purposely made an effort to teach any of my birds to talk, but they do to some extent, some more than other obviously...Quakers in-general are very good talkers, especially the males (that's typically how it goes with most species, but not always, there are always exceptions). My female Quaker, Lita, turned 2 years old 2 months ago in the beginning of May, and she has been talking since almost right after I brought her home at the age of 12 weeks old...Her voice isn't the clearest, but you can absolutely understand her. She says about 10 different full-phrases, such as "No Bites! Ouch!" (little smart-ass, lol), "Gimme Kisses", and "You're my Baby!", and she also does the most scary and spot-on "Evil Genius Laugh" I've ever heard a bird do, it's frightening...But I never purposely tried to teach her to say anything at all...My Green Cheek Conure, Bowie, is absolutely the best example of a bird who is not a species that is known for talking at all, and that I never expected to talk much, being one of the best talkers I've ever personally known, and again, I did absolutely nothing at all to teach him. His voice is not only completely clear, it also sounds like my voice, such as an African Grey can do accurately. Green Cheek Conures are definitely not known for their talking, and they often sound very raspy...But Bowie knows I don't even know how many words and phrases, dozens and dozens of them...And at just 2 years old this month (last week) he is still learning more and more words, seems like every week. He never ceases to surprise me when he spits-out a new word...Usually when he first starts saying a new word or phrase I have to ask him what he just said, as it's not very clear, so he'll say it again, I'll figure out what he's trying to say, and then i'll say it a few times, and then he's got it...It's amazing for a Green Cheek.
Yet, at the same time, the Congo African Grey that I grew-up with (my mom's bird, she got him when I was 9 and he was a just-weaned baby), who is now 32 years old, says very few words or phrases, and what he does say is not clear, he runs all of his words together and while we know what he is saying, strangers wouldn't be able to tell what most of his words and phrases are...He's the most intelligent parrot I've ever seen, by far, he actually tried to start the car once, no joke, I know that sounds ridiculous but he really did, I have another witness besides myself, my cousin, we were in the car waiting for my mom to come out of the video-rental store, key in the ignition and turned to the "accessory" position so the radio was on for us, and Ringo was sitting with me in the back. He crawled up into the driver's seat, which he did often because he was waiting for my mom, his "mommy", and I swear to God he perched on the steering wheel, grabbed the key with his beak, and cranked the car over...I'm not kidding...Anyway, so Ringo can start the car but he can't speak worth a damn, lol...
So the bottom-line is that some birds talk better than others, but it's never too late to teach them..It's all simply about repetition, that's how my birds inadvertently learned how to speak, not because I purposely tried to teach them, they just heard me say the same things to them over and over again and they picked them up...
If you really want to teach your bird to say a specific word or phrase, you just have to keep saying it to him/around him over and over as much as possible. It's no big secret or any special trick to it, just repetition. Some people make recordings of themselves saying the words and phrases they want their bird to learn, loop them, and play them all day long to their bird. They also make CD's and DVD's that do this, and I know there are specific App's you can download on your phone/Ipod/Computer where you can say the word or phrase you want them to learn into your phone or microphone on the computer, and then the program "loops" the word or phrase at your specifications, such as how many seconds elapses between the word or phrase being repeated, how long you want each individual word or phrase to be repeated until it moves on to the next one, etc. So you can try one of these App's/Programs and let it play during the day while you're at work, or for a certain amount of time each day, and see if it works for your bird...One tip, I'd concentrate on one individual word or phrase at a time, you'll probably be more successful that way...And once your bird starts saying that particular word or phrase clearly and on a regular basis, then you can move-on to a new word or phrase, but be sure to get him to repeat each new word or phrase he learns several times a day, so that it gets "stored in his memory banks", lol...