What do we know about how Marcus wrote the Meditations?

The τά εἰς ἑαυτὸν, "to himself" (which, though the work was almost certainly never given a title by Marcus Aurelius, is nevertheless the more proper ancient name of the text and the only one that appears in the manuscripts which give the work a title at all--Meditations is a modern title) was probably written, on the basis of references within the text, mainly in Marcus Aurelius' old age. The New Pauly suggests that it may have been composed mainly during the frequent visits to the Danube frontier that occupied much of Marcus Aurelius' last years, but that's technically speaking conjecture. The τά εἰς ἑαυτὸν is disorganized, confusing, and rather awkward, generally written with extremely compressed and awkward syntax and cannot have possibly been intended for publication or collection--only the first book has a coherent, if staccato and truncated, structure (the OCD suggests that perhaps it was composed independently, and certainly it bears indications of being an unrelated work). Whether the title that the manuscripts bear has any relation to a title that Marcus Aurelius himself gave to the work, it's a good description of the work--these were clearly memoranda, jotted down in free time and not intended to have any other readers besides the author himself. The preservation of the work appears to have been largely by accident, and we don't really know how it came to be. The work was unknown to Marcus Aurelius' contemporaries and there are no references to it until the 4th Century at the earliest--it's not clear what exactly Themistius is talking about when he refers to Marcus Aurelius. The division into 12 books (they're books, not chapters. Chapters mean something different in Classical literature) was probably not Marcus Aurelius', though it's not impossible. There are two main manuscript traditions, with some fragments floating around as well. The first, and the oldest surviving complete manuscript is the one being kept now in the Vatican. This text ultimately derives from the copies compiled by the Byzantine scholar Arethas around the 10th Century, who more or less rediscovered the τά εἰς ἑαυτὸν--we probably get the title from a brief remark in one of his letters referring to the work as Marcus Aurelius' notes to himself, which was passed on apparently to other manuscripts. The modern text as it's usually presented derives from the Codex Palatinus, which is now lost but was the source for Xylander's 1558/1559 edition of the text. In all likelihood the division of the work into 12 books derives from Byzantine scholars at some point, although as I understand it there's some confusion in the textual tradition as to how to classify the first book--in early editions it was often treated as a completely separate work, and it really is very different from the other 11 books.