A verse in Megilat Ester (9:32) refers to the Megila as a "Sefer" ("book" or "scroll"), a term that implies an association between the Megila and a Sefer Torah. On the basis of this association, the Sages established that the Megila used for the Mitzva of Megila reading on Purim must share the same basic physical properties as a Torah scroll. Specifically, it must be written on parchment prepared from the leather of a kosher animal, and the pages must be sewn together with "Gidin," veins taken from a kosher animal. As the Shulchan Aruch rules (991:5), a Megila whose pages are stitched together with some other material, such as flax, is invalid for use for the Mitzva.
The Megila should preferably feature an "Amud," or stick, at the end of the scroll around which the parchment is wrapped after the reading. The "Amud" should be attached to the parchment with "Gidin," the same material used for stitching together the pieces of parchment that comprise the Megila. Nevertheless, if a Megila does not have an "Amud," or if the "Amud" is sewn to the parchment with some other material, it is nevertheless valid for use, though it is preferable to have an "Amud" attached to the parchment with "Gidin."
Finally, the scroll should preferably have an empty column before the text of the Megila begins. This empty space should be large enough that it can be wrapped around the entire Megila when the Megila is folded.
(Based on the rulings of Chacham Ovadia Yosef in Chazon Ovadia Laws of Purim, p. 242)
Summary: A Megila must be written on pieces of parchment produced from a kosher animal that are sewn together with the veins of a kosher animal. It should have an empty column at the beginning, and, at the end, it should have a pole or stick around which the Megila is folded after the reading.
