Enter an online/offline game and make desired preparations for battle.  Walk over to the Battle Room Terminal and access the Computer.  Select "Map / Quest" -- if you're online, you'll be given an additional choice of "Free Battle" (you select map/rules/Story Characters), or "Random" (CPU selects map/rules/Story Characters).

Select a Map from the choices given (as pictured, there may only be one choice).  Select "Rule settings" to make changes (if allowed).  Next, go to "Deck selection" and pick the deck you'll use for yourself (and the Story Characters, if allowed).  Finally, select "Decide" to begin the match.  Note: not all decks are available for every battle -- some battles have Story Character class restrictions.

The highest roll determines which side/team gets to act first (pictured above-left).  You'll receive your first hand of 5 cards, and be given the option to use it or be dealt a new hand (pictured above-right).  As you can see, this is a one-time option given at the beginning of every battle.