Shulchan Arukh, Orach Chayim 691/שולחן ערוך·אורח חיים תרצ״א

1We may only write a Scroll with ink on a hide or on a parchment, like a Torah [scroll]. But if one wrote it with gallnut extract and iron sulfate, it is [also] fit. If he wrote it with other types of dyes, it is disqualified. And it requires scoring like a Torah itself. But its leather does not require processing for the sake of [the commandment]. And some say it (the leather) that it requires processing for the sake of [the commandment]...

2[If] it was written on paper or on hide that was not processed or was written by an idolater or a heretic, it is disqualified. The rules are the same as a Torah scroll with respect to empty parchment surrounding [each letter], the humps on a het, the disconnected hey and kuf, all the letters' shapes, and lacking or excessive letters Rema: It also has to be written based on a written copy [Ra"n]. [The scribe must] utter every word orally before writing it, just like with a Torah scroll [Mahari"k]. All of the sections are closed. If they were done opened, it is invalid [Hagahot Maimoni, piskei Mahara"i] After the fact, we do not invalidate a megillah based on lacking or excessive letters, because it is not worse than if the scribe left out some letters, in which case it is valid [Hagahot Maimoni, Or Zarua] There must be a pole at the end and a blank area at the beginning so that [empty parchment] surrounds it. Some say that it needs to have crowns, and some say that it does not. Rem"a: The practice is to use crowns. The practice is also not to put a pole at the end at all [Mahari"l].

3[The section with the names of] the ten sons of Haman should be written like a song, but not like the other songs where there is blank space next to the writing. Instead, one leaves blank space between each word. If he did not do this, it is invalid.

5If it was sown with linen thread it is disqualified

8We do not read within a community from a scroll that is written among (together with) the Writings, unless [its pages] were bigger than the other pages or smaller [than them] — so that it will have a distinction. But an individual can read it — even if it is not smaller or bigger — and fulfill his obligation with it, provided it is written in a column like a Torah scroll

9If one read from a stolen Megilla scroll he has fulfilled his Mitzvah.