Instructions
Collect at least 30 different Pokemon for your Pokedex. Go to the eastern end of Route 11 where it connects with Route 12 to locate Professor Oak's Aide. He will give you an item finder if you have a sufficient number of Pokemon.
Go to any location where you'd like to look for hidden items. Caves and sandy areas generally have hidden items, but they are scattered in hundreds of different locations. Press "Start" to access the Menu screen.
Scroll through your available items and select the item finder. Press "A" to confirm your selection. The item finder will automatically look for hidden items on the screen. Two pings followed by two arrows means that you need to go two or three steps in the indicated direction, while four pings followed by four arrows means that you're right next to the hidden item.
Press "A" in the direction indicated by the arrow. This will automatically initiate digging if you're next to the hidden object. If not, use the item finder again to pinpoint the hidden location, then press "A" again to dig.