In this one I first composed the melody for the horns and then harmonized it with the basic triads below. It's a great way to beef up a melodic statement without resorting to adding more instruments to a unison line. Best used for shorter passages where you want that extra energy, can sound out of place if overdone.
The last four notes of the Star Wars theme by you know who is a great example of how to do this tastefully. Any excuse to listen to some Williams is a good one so go check that out here, it's at 0:16 and 0:27.
Try it yourself, take either an existing melody or write a new one. Add the "missing" triad notes below the melody. See each melody note as either 1, 3 or 5 of a major or minor triad. Experiment and see what you come up with, there's a lot of different combinations to explore.