Pokeblocks are needed to max out your Omega Ruby and Alpha Sapphire Pokemon for contests, and this guide will tell you everything you need to know about making them.
Go to the berry field on Route 123 and harvest the berries. Berries which reduce super effective hits make great choices for Pokeblocks, so plant any of those you might have. Many good berries are given to players each day throughout the Hoenn region.
Plant these berries as soon as you can, as you're going to need a lot of them to make enough Pokeblocks to max out your Pokemon.
To make a block, just go to your items and use the Pokeblock Kit. Then choose between 2 to 4 berries to make 2 to 4 Pokeblocks, with the berries you choose yielding different Pokeblocks. A strong Pokeblock created using more powerful berries will greatly enhance the abilities of your Pokeblock. A rainbow Pokeblock can be made by mixing four different types of berries together to max out the various stats faster.
You can watch the Pokeblock guide below from Verlisify.
