Shulchan Arukh, Yoreh De'ah 288/שולחן ערוך·יורה דעה רפ״ח

1With what, and how, it is written.How is the mezuzah written? He writes two sections on one pane (daf) in a single sheet (yeri`ah): Shema` and Vehayah im-shamo`a. He leaves a space below and above, approx. half a pinkly-fingernail-width (tziporen). When beginning he should measure in order [to leave a top margin] to enable unrolling it after it's rolled. Rem"a: At the bottom he needn't leave any margin. (Tur)

2It should be written in one [column] (daf) but if he wrote it on two or three [columns] it's kosher.

3If he wrote out of order, even if he forgot one letter, it's invalid and irreparable; needless to say if he put the wrong paragraph first.

4If he wrote it on two parchments even if he stitched them it's invalid.

5A priori the hide needs to be made for-the-mitzvah (lishmah). But where if he'll wait for hide that's made lishmah he will be delayed to do the mitzvah, then he should write it on available hide that is not made lishmah and perform the mitzvah immediately, while requesting hide that's made lishmah.

6Whether written on qelaf (split leather) or gewil (unsplit leather), it's kosher. They did not restrict using dukhsustus (an inferior half of the split), but instead were saying the optimal mitzvah [is done on the aforementioned]. (Shabbat 79b) (And our qelafim are kosher by all opinions.) (Mordekhai halakhot ketanot)

7Its rules regarding the ink, writing, crowns of the letters Sh`ATNeZ GaTz, are same as for sefer torah.

8[The parchment] requires scored lines. If written without scoring, it's invalid.

9He writes all its lines equal-length; one should not be longer than another. If one line is longer than the previous or the next, or shorter than the previous or next, it's kosher. As long as it's not arched, or like a tail, or round.

10`Al-ha'aretz/on the Land should be written at the head of the last line, and write nothing more after.

11The custom is to write it in 22 lines.

12It needs to be written with the right hand, like tefilin.

13The space between paragraph Shema` and paragraph Vehayah im-shamo`a, it is a mitzvah to make it a closed space. If he made it an open space, it's kosher, since it's not adjacent in the Torah.

14It is rolled from end to beginning, from Echad to Shema`.

15It is forbidden to add anything to it, except on the back side we write ShaDaY opposite the word Vehyah on the front.Rema: Some say opposite the space between the paragraphs (Tur in the name of RO"Sh, and the Short Mordekhai and Rambam and HGH"M [Hagahot Mordekhai] in the name of SH"T [Sefer haTerumot]). And that is the custom. And we leave a window in the case opposite the name ShDY so that it's visible from outside (there, in Mordekhai and HGMY"Y [Hagahot Maimoniyot]). We also have the custom to write on the outside KUZU BMUKS"Z opposite H' Eloheinu H', and write these letter upside-down versus the mezuzah writing, so that each letter touches opposite its replacement letter in the Alef-Beth (Hagahot Maimoniyot). But inside, add nothing at all besides the two paragraphs. (Tur and Ramam)