andybc
Even if her exact method was revealed, I do doubt it would work for others. Everybody has their own personal methods. However, aside for being quite intuitive/psychic (which plays a big part) she was extremely adept in many forms of divination (predictive astrology, palmistry, cartomancy) and if one uses all of these different methods in a reading, they will likely excel and provide astounding accuracy.
Le Normand was not a palmist at least not in the true sense. She would look in a book to find a hand that seemed most like her client's which is attested by her clients. IN one of her books she answers a question about Tarot by quoting a palmistry text. Thortly after her death a deck, based on these books which were common at the time, appeared in France.
We also do not know if she was an astrologer. She uses the term horoscope in her own texts to refer to card readings, and she would write up her client's readings as a 'horoscope'.