Most popular accounts of the I Ching describe sixty-four hexagrams and stop, which makes it a book of sixty-four fortunes. That is not what it is. The mechanism that makes it a system is the changing line.
A consultation produces six lines, one at a time, and each line comes out in one of four states: young yin, young yang, old yin or old yang. Young lines are stable. Old lines are changing, they are read in their current form and then flip to their opposite, producing a second hexagram. A reading is therefore normally two figures and a movement between them: a situation, and what it is turning into. The texts of the changing lines are the specific advice; the two hexagrams are the frame.
The method matters, and this is where most modern practice quietly departs from the old one. The yarrow stalk procedure, the older method, does not produce the four line states with equal likelihood: old yang comes up markedly less often than old yin, and the whole distribution is lopsided. The three-coin method, which is faster and now far more common, makes changing lines of both kinds equally probable. The two methods therefore ask different questions of chance, and a book of readings compiled under one does not straightforwardly apply under the other. This is a real and under-discussed difference, not a scruple.
There is also no agreed rule for what to do when several lines change. Various conventions exist, read the lowest, read the highest, read them all, read only the second hexagram, and they descend from different commentators. The earliest records of consultations do not use a single system either.