First you'll have to meet with Maechen at Guadosalam in Chapter 3. You'll find him when you meet up with Logos and Ormi in the same area. Next you have to get the ''whistling'' scene with Yuna at the end of Chapter 3. When you're alone in the Farplane, move Yuna around and continually press the X button. You'll hear whistling. That means you have activated the event. Next thing you have to do is get an Episode Complete in Luca during Chapter 5. All you have to do there is visit the location where Tidus taught Yuna how to whistle. Next, get an Episode Complete in Besaid. Finally, once you have beat the game, keep tapping the X button to get the ending.