THere's not much to explain. You can have no more than one of any item on your team, and no more than three of your POkemon can be from the OU or BL tiers.
Ok that's good. Understand that should you break a rule I have the right to ask you to flee the battle (not D/C). Should you deny or refuse you get banned from the league. I also reserve the right to a trade after the battle to check your Pokemon.