You may get the 'conire' (berry jar) from a man in his house north west of the Pokemon center in Cerulian City.
Now, in order to Berry Crush you’ll need to visit any Union Room in the game using the Wireless Adaptor.
The way you do it is exactly like the ‘Pokéblock Blending’ in R/S – simply pressing A at the relevant time to make the little hand move faster and faster, thus crushing finer Berry Powder. The better and faster you crush the berry, the more points the Berry Powder will be worth.
The point of Berry Powder is to obtain high points. After the Berry Crush, you will go back to the man in Cerulean City who gave you the Powder Jar, and he’ll offer you a variety of items to trade your Berry Powder for – a bit like Buena’s Password thing in Crystal.
Check out great tips, and hints for Pokemon - FireRed on Game Boy Advance from those who play the game at Supercheats.com.