Each card in a tarot deck has multiple meanings; usually it'll have several for when it's upright and several more for when it's reversed. In addition to those standard meanings, each deck may have slightly altered or entirely new meanings for some of its cards, and certain readers who have used the same deck for a long time will ascribe meanings all their own.
For example:
I have a deck called "Tarot of the Cat People". It's not a deck that I actually use. For 10 swords upright, it says:
Vigilance. Agility. Spying. A discreet person. An active youth. A lithe figure. This card symbolizes a percen adept at percieving... <SNIP>
Then for reverse meanings, it says:
An imposter revealed. Illness is also possible. Powerlessness in the face of stronger forces...<SNIP>
Other decks define this card in similar, yet different terms.
In my personal experience, every time in the last two years that I've used my Robin Wood deck, and the 10 of swords has come up, someone has died. I've predicted four deaths in my friends' immediate families and two in my own. I actually called my father and told him to go and call his own father. To tell him that he loved him and get things in order, because I was worried that grandpa was going to pass away sometime within the next two years. A year and a half later, he did.
So how do you know the meaning of the cards? That's due to the spread and the relationships between the cards. People often create their own spreads, and so these meanings can vary.
Another thing to consider is that someone who specialized in past life regression may not use a standard tarot deck, as this isn't tarot's specialty. They may have actually created their own deck. In this case, the cards, meanings, layout, and technique would all likely be unique to that particular reader.
That may be the best way to go. If, for instance, someone who owned the Tarot of the Cat People were reading your book, and you had 10 swords representing death, the person who owned the Tarot of the Cat People may scoff and say, "That's not what that means!" With so many meanings for each card, and those meanings being affected by so many things (specific deck, reader, questioner, card position, ect), your best bet may be to invent an entirely new deck.
If you want to invent a new deck, then research the basic meanings of the tarot. The tarot has four basic suits, one representing each element. These four also represent the four different directions. They go from one to ten, and these can basically be described as a journey. Ones are often beginnings, tens are often culminations. After one-ten, you have Page, Knight, Queen, King. These can represent people. Pages often represent messages. They are often youths. Knights can be the coming or going of a matter. They are often older, but not yet fully mature. Queens and Kings are very representative of the values particular to their suit.
Then you have the Major Arcana. They also go on a journey, from 0 The Fool, which I was taught was "The Uninitiated Soul", to 21 The World, which I personally think of as "All the Good Things in the World". These Major Arcana are the heavy-hitters of the deck. They can deal with Karma, the Gods, things out of the questioners control. They are the Forces that Be.
If you design your own deck with these things in mind (you'll need to do some more research; I only hit on the very basics) then your deck will ring true to those who know the tarot.
Then design your tarot reader, layout, and questioner in such a way that the audience can almost "see" what the cards are saying.
That would be my advice, as obtuse as it is.
