To begin this little quest - which, despite being a Side Story, is somewhat crucial to the main questline - you'll need to proceed far enough into Nanohana that you're headed to the Desolate Valley. It's located in the Desert Near Nanohana, to the west of town. Get through Desolate Valley and you'll meet a traveler in a cave who will tell you that you need a Navi Bird to go any further.
Head back to Nanohana and look on the western outskirts of the city, in the Exit to Desert area. The Navi Bird is sitting on a post out here, and it will offer to help you get through the Ravine of No Return, your next destination, if you feed it Desert Nuts. There's only one way to get Desert Nuts, and you'll need Usopp's help to do it.
Head into Nanohana and check along the rooftops. It won't take long before you spot a bird's nest, gleaming away above your head - and well out of reach. The process for snagging them is pretty easy:
- Put Usopp in the lead position of your party
- Aim and fire at the nest you've found - targeting them is identical to grappling / grabbing distant items with Luffy
- Run over and grab the fallen Desert Nuts off of the ground