Writing a Sefer Torah
There is a positive mitzvah to write a Sefer Torah learned from the pasuk "ועתה כתבו לכם את השירה הזאת". Some explain that the mitzvah of writing a Sefer Torah is supposed to facilitate learning Torah.
- Writing or fixing just one letter in a sefer torah is a fulfillment of this mitzvah.
- Buying an already written sefer torah according to some opinions is a fulfillment of writing a sefer torah, while according to others it isn't a fulfillment of the mitzvah.
- Some say that buying Sefarim is a fulfillment of this mitzvah. Additionally, one shouldn't sell a sefer except in order to learn torah or to get married.
- Writing a Sefer Torah together with other people is a fulfillment of the mitzvah for everyone involved. Others hold that one doesn't fulfill the mitzvah in partnership.
- If someone else writes a Sefer Torah on someone else's behalf without their knowledge the owner isn't fulfillment of the mitzvah.
- If one can't write the Sefer Torah himself he should hire a sofer to write it for him.
Who is obligated to write a Sefer Torah?
- Men are obligated to write a sefer torah.
- Women aren’t obligated to write a sefer torah. If a women writes a Sefer Torah it is invalid.
- A sefer torah may only be written on the hide of a kosher animal or bird. It may not be written on the hide of a non-kosher animal, bird, or the skin of any fish.
- A sefer torah may be written on the hide of animals that are from kosher species even if that animal itself is a Terefah or Nevelah. 
See the Tzurat HaOtiot page for How To Write Letters in a Sefer Torah, Tefillin, and Mezuzot.
- Yemenite Sifrei Torah have symbols for the end of a Pasuk, for example, pressed in to the parchment with a metal instrument. This way, the reader is informed of certain stopping points and pronunciations. Such scrolls are even Kosher for Ashkenazim to read from, but only Yemenites should be exercising this practice. Bediavad, Ashkenazi Sifrei Torah would also be Kosher, but it's not recommended Lechatechilah.
- Rambam (Sefer HaMitzvot Asin #18), Sefer HaChinuch (n. 613)
- Devarim 31:19
- Rosh (Hilchot Sefer Torah n. 1)
- Rambam (Sefer Torah 7:1)
- Rashi Menachot 30a s.v. Kechotef and Rambam (Sefer HaMitzvot Asin n. 18) imply that one does fulfill the mitzvah when one buys a sefer torah. However, the Rama YD 270:1 writes that one does not fulfill the mitzvah. Gra YD 270:4 agrees with Rashi that it is a fulfillment of the mitzvah.
- Rosh (Hilchot Sefer Torah n. 1) writes that nowadays that the sefer torah is left in shul for public torah reading, there is a mitzvah for every Jew to write a chumash, mishna, gemara, and commentaries in order to be able to learn Torah. This codified in Shulchan Aruch YD 270:2.
- Bet Efraim YD 63 writes that if everyone in a group writes a letter of a sefer torah they fulfill the mitzvah but if they all jointly hire someone to write a sefer torah for them it is questionable if they fulfill the mitzvah. The Pitchei Teshuva YD 270:1 cites the Bet Efraim as well as the Pardes Dovid who says that the minhag is to write a sefer torah in partnership. Rav Moshe in Igrot Moshe YD 1:163 argues that it should fulfill the mitzvah.
- Aruch Hashulchan YD 270:11
- Pitchei Teshuva YD 270:7 quotes the Mishnat Chachamim who says that if someone else writes a Sefer Torah they wouldn't fulfill the mitzvah since according to the Rama buying a Sefer Torah doesn't fulfill the mitzvah and according to Rashi it isn't the optimal mitzvah and there's no zachin for something which isn't a benefit.
- Rambam Sefer Hamitzvot 18. Mishneh Halachot 16:101 argues that perhaps writing a Sefer Torah is a mitzvah upon a person's body and if a person can't write it hiring a scribe to write it for them is only a fulfillment of a partial mitzvah.
- Rambam (Sefer Torah 7:1), Shulchan Aruch YD 270:1
- The Rambam writes that the mitzvah of writing a sefer torah does not apply to women. Chinuch (Mitzvah 613) agrees. The Shagat Aryeh (Siman 35) questions this. He explains that the understanding behind the mitzvah of writing a sefer torah is that a person should prepare a sefer torah for himself so that he’s able to learn from it. If so, since women need to learn the laws that are relevant to them, they should be obligated to write a sefer torah. The Bet HaLevi 1:6 answers that fundamentally women aren’t obligated to learn torah. While it is true that they are obligated to learn about the mitzvot that they are obligated in, that’s only as a preparation of fulfilling the mitzvah, but not as a part of the mitzvah of learning torah in general. The obligation of men to learn torah is all encompassing; they are obligated to learn all of torah, even about the mitzvot that aren’t relevant to him. Chida in Shiurei Bracha 270:2 rejects the Shagat Aryeh in favor of the Rambam and Chinuch.
- Gemara Menachot 42a, Shulchan Aruch Y.D. 281:3
- Shulchan Aruch YD 271:1. Gemara Shabbat 108a has an unresolved question as to whether one may use the skin of kosher fish for the writing of a sefer torah. The Shulchan Aruch YD 271:1 rules that one may not use fish skins even of kosher fish.
- Briatta quoted on Gemara Shabbat 108a, Shulchan Aruch YD 271:1
- Shu"t Revid HaZahav based on Shu"t HaRivash (Siman 286) and Rama (Yoreh De'ah 274), Shu"t Iggerot Moshe Yoreh De'ah vol. 3 Siman 117