Just find the itemfinder press A and select 'use' then you're using it!
In the gate on Route 11, east of Vermillion City, you will find one of Professor Oak's aides. He will offer you the Itemfinder when you talk to him, but only if you have at least 30 Pokémon on your Pokédex.
In Generation I (Red, Blue and Yellow), the itemfinder is obtained from Oak's aide in building at Route 11. You must have caught at least 30 different Pokemon before he will give it to you.
There small, they look like R's, the have a yellow circle at the end that blink when they start finding an item.
when you have got the itemfinder. Click start then click on bag and then press the right button on your +control pad. click on the itemfinder then click use. The itemfinder will detect any items around you that are berried.
Press use to use the itemfinder and if it dings you have found something. Walk in the direction you are facing then use the itemfinder again. continue until the itemfinder starts shaking wildly and press A facing the spot and you will find something.
somewhere
If you have the itemfinder app for the pokech, go by the bottommost building and use the itemfinder.
The itemfinder will point you in the direction of where hidden items are press A in that direction to find the item.
Simply press the A button in the direction the itemfinder is pointing to.
You can get all 4 shards, red, yellow, green and blue. you can find them on islands or underwater spots. Sometimes, they will be invisible, so use an Itemfinder. Thanks!
Pokemon emerald