Instructions
Select the "PokeWalker" option from the game's main menu. Hold the PokeWalker up to the infrared sensor of your DS, about an inch away from the sensor. Choose which Pokemon you want to take for a walk; you can take up to three of the Pokemon in your party. Choose the course you want to walk on; when you start out, you will be limited to the Noisy Forest. Clip the PokeWalker onto your shirt or belt loop; the PokeWalker will track how many steps you take. Take your Pokemon on walks as often as you can. You need to earn 100,000 watts to unlock the Quiet Cave.
Unlock the Quiet Cave course once you have earned enough watts; when you use the stylus or "A" button to tap the "PokeWalker" option in the main menu, select which Pokemon you want to walk, then tap "Quiet Cave" to choose the course.
Walk around in the Quiet Cave course with your Pokemon. Wild Pokemon will be hiding in the bushes; an exclamation point will appear over the place where Pokemon are hiding. Tap "A" on your control pad to engage the wild Pokemon. When you encounter a wild Feebas this way, you can attack it to whittle down its health, then throw a Pokeball to capture it.