After each fight, roll a d14-1, even though everybody knows d14's don't exist. Then roll a d2 (or flip a coin, whatever) to determine which way the card is turned.
Right way round, it can be used on an ally. Upside down, it has the opposite effect on an enemy, unless noted.
Bosses might be immune to some effects, though.
0,
The Fool: The fool represents limitless possibilites. Your target's next roll receives a +1 bonus.
1,
The Magician: The magician represents action and self confidence. Your target gets a +1 to hit and damage on their next subweapon attack.
2,
The Priestress: This card represents hidden knowledge and wisdom. Your target regains 20% of their MP.
3,
The Empress: The empress represents motherliness and... uh...
other qualities commonly associated with women. Your target gracefully dodges out of harms way with a +2 def bonus.
4,
The Emperor: The emperor stands for tradition and leadership. Your target gets a +2 bonus to hit for an attack.
5,
The Hierophant: The pope stands for religion, natch.
Greenmore turns into the Super-Saint. You heal 20% HP. If upside down, it deals 20% holy damage.
6,
The Lovers: This card means that you have a decision to make. However, we're gonna totally ignore this interpretation and substitute it with our own. Two targets of your choice split any damage they receive between them.
7,
The Chariot: The chariot stands for conquest.
Your target charges into battle before anybody else does with +15 on initiative.
8,
Justice: Guess what this represents. If an enemy hits you in melee, you counterattack. If used on an enemy, their next target, if in melee range, can retaliate.
9,
The Hermit: The hermit is a paragon of wisdom. Your target can shift out of battle, making it so that enemies ignore you for an entire round. They cannot act though, but allies can still heal or buff them or whatnot.
10,
Fortune: You feel lucky! Your target rerolls their worst roll on their next turn, and take the better result.
11,
Strength: This arcana is the STRONGEST! Your target gains +1 for hit and damage on their next regular attack.
12,
The Hanged Man: The hanged man represents acceptance and sacrifice. Your target draws an attack towards themselves in an attempt to save their friends. An enemy gets stunned for the next round.
13,
DEATH: Even though most interpretations are
exactly what it says on the tin, this arcana stands for change and a new beginning. Your target recieves +50% extra damage for the entire fight. If it's an ally, and they die, they remain in battle for one more turn with a +2 modifier on all rolls.
The fool, priestess, and hierophant can also be used outside of a battle, but I still have to wait until after the next encounter to pick a new card.