The original 1878 edition included, in lieu of a preface, an inspiring paragraph from Descartes' Discourse on Method. My sense is that Nietzsche included this not only because it makes a substantive point about philosophy, but also because it is illustrative of the sort of free spirit he is discussing. Descartes has a task and a strong will to pursue it, and as a result "all else seemed of no account."
The preface under discussion is therefore that of the 1886 edition. It is primarily an explanation of why he created the notion of the "free spirit" and the evolution of the psychology of such a person. Coincidentally, I just recently read Ken WIlber's The History of Almost Everything, and the influences on Wilber (which he acknowledges, though not with specifics) are manifest. This is most apparent in the emphasis on the word hierarchy. Based on Nietzsche's frequent reference to health and to isolation, as well as the fact that he explicitly states that there are no such free spirits - that this is actually a personal account of his own struggle,
Section 1 somewhat pridefully recounts the reactions readers have had to his work, in particular his challenging of virtually all established beliefs and practices: "I myself do not believe that anyone has ever looked into the world with such deep suspicion." He goes on to point out the personal cost this sort of stance has, and justifies a degree of artistic license ("counterfeiting") because he needs it. This is best captured in his parenthetical: "And what have poets ever done otherwise? And why else do we have all the art in the world?"
In particular what he needed was kinship, the idea that someone agrees with him, thus in Section 2 he explains that this is why he invents - as opposed to merely describes - the "free spirit." They do not yet exist, but he believes they could, foresees that they will, and hopes that his efforts here encourage them.
Section 3 describes the initial flowering of a free spirit; the first step he refers to as the "great separation." This is a sudden event in which one rejects one's obligations, traditions, and past - and seeks out the new, the forbidden, the dangerous. It feels to the subject like a great, first victory, though the nature of that victory is unclear, and it leads to taking great risks as the individual first truly exercises his freedom and will. And, by asking such questions as "Cannot all values be overturned?" finds himself isolated and therefore lonely. It is much like a modern account of teenage rebellion, though referring to a distinctly intellectual rebellion.
In Section 4 Nietzsche attempts to describe an intermediate stage, with a long distance still to travel, on the path to achieving "mature freedom of spirit" with its "superabundance." In this present stage there is still risk of settling on some particular and narrow set of beliefs rather than being able to view the world from many different perspectives. The metaphor - or perhaps it is not merely a metaphor - of health is used repeatedly here, and during this stage there are many painful steps during which health seems present, but perhaps is not. By the end of this stage, "no longer chained down by hatred and love, one lives without Yes, without No..." This seems to be a description of the emotional content of what a thoroughgoing perspectivism brings.
Section 5 describes a stage of self-analysis - looking at oneself, back at one's past, and at one's intellectual and emotional errors ("ill health") with detachment and even pleasure, to truly see and understand what one is and has been. The progress is slow and incremental but this adds to the wisdom one gains.
In Section 6 the now-relatively-developed free spirit looks further back at the great separation itself and asks why (since it was so costly) - and actually comes up with an answer. Nietzsche describes those things that one had to do or learn - gaining mastery over virtues, control over the "For and Against," and "learn that all estimations have a perspective," which leads to injustice, which is inherent in life and determines it. Life necessarily is the purpose and measure of things and self-preservation drives its perspectives and choices. Finally, the free spirit grasps the problem of hierarchy though it is not really elaborated here.
Section 7 has the free spirit recognizing his task, his destiny, which is the problem of hierarchy, and only now does he fully realize that all the tribulations explorations of his inner world have been necessary to prepare just for recognizing that as the problem, which is a new problem. "Here is a Higher, a Deeper, a Below-us, an enormous long ordering, a hierarchy which we see..."
Section 8 points out that a psychologist can see where these stages of development fit in, but laments that there are no real psychologists in Germany, though there are elsewhere in Europe; and explains that the book (the first edition) was not well-read in Germany because "it demands too much," in particular a state of mind and set of sensitivities that Germans do not currently have.