You can approach this quest only after finishing the main quest The Horn of Jurgen Windcaller. Enter High Hrothgar and find Arngeir. Tell the priest that you want to learn something more about the Voice and ask him of any new Words of Power (screen above). Arngeir will therefore send you to find a Word of Power to a randomly chosen location. You can repeat this quest multiple times. If Arngeir won't have any new locations for you, wait a couple days and speak to him again. In case you receive nothing again, it might mean that you have learned all the "widely available" Words of Power.