The Shulchan Aruch (32:7) rules that the parchment used for writing Tefillin must come from the part of the hide called the "Kelaf," rather than on the "Duchsustus" or "Gevil." "Kelaf," he explains, refers to the layer of skin closest to the animal's hair, whereas "Duchsustus" refers to the inner layer, closer to the flesh. "Gevil," according to the Sulchan Aruch, refers to the two layers of skin together, before they are detached. It was taught through Halacha Le'Moshe Mi'Sinai (a tradition conveyed orally to Moshe at Sinai) that the Tefillin must be written specifically on "Kelaf," the layer of skin closest to the hair, after the two layers are separated. Furthermore, the Shulchan Aruch requires writing the Tefillin on the inner side of the "Kelaf," namely, the side that faced towards the animal's flesh.
Before the "Kelaf" becomes suitable for use as Tefillin, it must first undergo a process of tanning using either gallnuts or limestone. In ancient times, it was very common for people to use gallnuts for treating leather, whereas nowadays limestone is used for this purpose (Shulcha Aruch 32:8). Preferably, he should preferably say "LeShem Tefilin" before he starts tot an them.
Some Soferim (scribes) apply shellac to the parchment before they begin to write the text, a process referred to as "Mashu'ach." The shellac lends the parchment a shiny appearance and makes the surface smoother, allowing for easier writing. For a variety of reasons, the Halachic authorities discouraged the use of this kind of parchment. According to some views, the shellac constitutes a "Chatzitza," or interposition, between the parchment and the text, thus disqualifying it for the Mitzva. Furthermore, it is virtually impossible to correct cracked letters on this kind of parchment, and thus the Sefer Torah, Tefillin or Mezuza would have to be replaced if a letter begins to crack. It is therefore preferable to purchase one's Tefillin, Mezuzot and Torah scrolls from a scribe who does not apply shellac to the parchment, even if this adds to the cost. Nevertheless, one who does use Tefillin, Mezuzot or a Sefer Torah prepared in this fashion has nevertheless fulfilled the given obligation. (See Halacha Berura, Helek 2, page 235.)
Finally, the parchment used for these Mitzvot must be taken from a kosher species of animal. The Sages derived this requirement from the Torah's comment in reference to the Mitzva of Tefillin (Shemot 13:9), "in order that the Torah of God be in your mouth." This verse suggests that Tefillin must be made from materials that may "be in your mouth," meaning, that are permissible for consumption, and hence the parchment must be taken from a kosher species of animal. The animal need not, however, have been properly slaughtered for its hide to qualify for use as parchment; similarly, one may use an animal's hide for parchment even if the animal is discovered to be a Tereifa (suffering from a fatal illness, in which case it is forbidden for consumption). So long as the animal belongs to a kosher species, its hide may be used as parchment for Tefillin, Mezuzot and Sifrei Torah. (Ibid 12)
Summary: Parchment used for Tefillin, Mezuzot and Torah scrolls must be taken from a kosher species of animal, and from the outermost layer of skin, near the hair. The text is written on the inner side of that layer of skin. The piece of parchment must first be treated with either gallnuts or limestone. Halacha discourages using Tefillin, Mezuzot or Torah scrolls that are "Mashu'ach," meaning, upon which the scribe applied a layer of shellac before writing the text.
