Also note that we are adapting from the Cthulhu by Gaslight supplement. The major affect of this seems to be that characters start out with a much higher default Credit Rating score.
To use them, the player shows the card, and describes a result which should happen that is in line with the interpretation of that card's meaning. The intepretations will come straight from the Waite deck. For example, ``Victory'' is a pretty standard interpretation for 6 of Rods -- a player could use this card to succeed over an enemy. As long as everyone more-or-less agrees that the results is (1) plausible, and (2) fitting for the card, then what the player describes will happen.
The Major Arcana are more powerful, and in some confused way relate to forces actually present in the game-world. Thus when one is used, the player must make a POW roll for his character: essentially for control of the forces at work. The player then describes to the GM what is meant (as above), and the force in question enters the situation. Of course, an 01 POW will mean that the interpretation you put on it is almost certainly going to apply; an 00 is going to drop you in the shit.
The GM's idea is to make these the basis of the magic in the game -- which like real alchemy and tarot function in a symbolic and metaphorical manner (as opposed to concrete spells which PC's can cast). Previously, it seemed like the GM's control was overwhelming. Magical activities by the PC's were ultimately limited to ``I make a POW roll.'' -- which the GM would then interpret.
The tarot cards were intended to give the players more of a feeling of control over what was going on -- as well as representing the magical forces which were prevailing. As the GM put it, ``The idea is that you can simply ignore what I want to happen and insert your piece of weirdness, and then make me deal with it.''
For more information on Tarot, see the Links section of the tarot page on this site.