Instructions
Collect berries. You can find them growing on bushes and trees throughout the land of Hoenn.
Travel to Slateport City and speak to the little girl in the Contest House. She will give you a PokéBlock Case.
Go to a PokéBlock Making Machine, which looks like a grey square with red dots on it. When a dialogue box opens up, select "Yes" until you see a list of your berries.
Choose any berry you want and confirm your selection. You will now be brought to a mini-game with a spinning dial in the middle of the circle, along with your name and the other participants on each corner of the screen.
Press the "A" button whenever the dial spins on your name. You will get a red "O" symbol or a bull's-eye if you timed your "A" button press properly. Otherwise, you will get an "X" symbol.
Finish playing the mini-game. Afterwards, you will receive your PokéBlock, which will be placed in the Important Items Folder of your PokéBlock Case.