- One must tear Keriyah while standing. If one did it seated, one didn't fulfill one's obligation and so, one should tear one's garment again without a bracha.
- A elder or disabled person who can't stand, should tear Keriyah while seated. 
Where on the garment to rip
- One should tear the front of the shirt from the collar downward (vertically) and not across. If one tore the back, the bottom, or the side one didn't fulfill his obligation.
- One must tear the garment where it was originally complete and not an area which was only sewn together. 
- One who is mourning a parent should tear on the left side of one's body so as to uncover one's heart, but one who is mourning any other relative should tear on the right side of one's body. If one did otherwise, after the fact one fulfilled one's obligation either way.
How to do the tear
- For a parent one must tear with one’s hands and for another relative one can tear with one’s hands or with knife. Even for a parent the minhag is to start with a knife and then the person tears downward a tefach.
- The Sephardic minhag is that someone else does the kriyah for the mourner and he recites the bracha.
Which Garments to Tear
- One who is mourning any relative other than a parent should tear should tear one's outer garment such as one's shirt, but not an outer garment which one only wears sometimes such as a coat or jacket. 
- One who is mourning a parent should tear one's outer garment as well as any undershirt that one is wearing until one reveals the skin covering one's heart. One does not need to tear a outer garment which one only wears sometimes such as a coat or jacket. 
- If one only tore some of the garments which one was obligated to tear, one has not fulfilled his obligation. 
How far to tear
- If one is mourning any relative other than a parent one should tear the length of a Tefach and not more because of a concern of Baal Tashchit. 
- One who is mourning one's parents should tear until the skin covering one's heart is revealed. 
- If one is wearing an already torn garment when one is tearing kriyah for a relative other than a parent it is sufficient to rip a little bit unless he is still in the middle of shiva mourning for the relative from whom he tore the shirt the first time, in which case one has to rip a tefach for the other relative.
When to tear Keriyah
- Ideally a person should tear kriyah immediately or as soon as possible after the person dies or he hears about it. The minhag, however, is to tear kriyah after the burial or before the seudat havrah. This minhag has what to rely upon.
- If someone's relative died he doesn't tear keriya on Yom Tov even the second day.
- On Chol Hamoed it is permitted to tear kriyah for a relative for whom one has an obligation to tear kriyah.
Keriyah for women
- A woman who is mourning a parent should tear Keriyah on the inner garment first, turn that tear to the side, and then tear the outer garment so as not to reveal the skin covering her heart.
- While the minhag of some Sephardi women is not to tear Keriyah at all out of concern for tziniut, many poskim hold that this is in error.
For whom does one tear Keriyah
- For any relative for whom a person is supposed to mourn he must tear kriyah. These include: A mother, father, son, daughter, brother, sister, and wife.
- Additionally, if one of the seven relatives are in mourning one should tear kriyah before those relatives to join them. The Ashkenazic minhag is not to tear for a relative's relative today.
A Good Person
- Someone who has a good reputation, isn't suspected of any aveirah or omission of a mitzvah, even if he isn't great in torah one should tear kriyah if he knew about it between the death and the burial. The Ashkenazic minhag is not to tear kriyah on a good person unless one was present at the death.
- For a chacham or talmid chacham who could answer a question of halacha in any area of torah one should tear kriyah when one hears about his death and if one missed that then when they eulogize him. This kriyah is until one reveals one's heart. Ashkenazim hold that this only applies to a talmid chacham who one learned from.
- The minhag is not to tear Keriyah upon hearing about the passing of a Talmid Chacham unless it is one's Rebbe Muvhak.
- A rebbe muvhak is someone who whom you learned the majority of your learning whether it is in tanach, mishna, or gemara. This kriyah is until one exposes one's heart and some say one tefach.
- The minhag is not to tear kriyah for a chevrusa from whom one learned from.
Patient at time of death
see Nishmas Avraham vol 1 oc 223:6 and vol 2 Y.D 340:2 for lengthy discussion
- While the Talmud and the Shulchan Aruch clearly state that a person is obligated to rend one’s garments when present at the passing of another Jew, the common practice is to be lenient, especially for doctors and nurses.
- One who does rend his garments when present at the time of death need not tear a full tefach for each passing, rather one tefach for the first and then add on a little bit (kol shehu) for each subsequent passing.
- Kitzur S”A 195:1. The Rosh Moed Katan 3:44 quotes the Raavad as holding that tearing keriya is biblical, while the Ramban argues that it is only rabbinic. The Shach 340:2 cites the opinion of the Ramban.
- Shulchan Aruch 340:1, Kitzur Shulchan Aruch 195:1
- Rav Mordechai Eliyahu's comment on Kitzur Shulchan Aruch 195:1
- Rav Mordechai Eliyahu's comment on Kitzur Shulchan Aruch 195:1
- Shulchan Aruch YD 340:2, Kitzur Shulchan Aruch 195:2
- Kitzur Shulchan Aruch 195:2
- Kitzur Shulchan Aruch 195:4
- Kitzur Shulchan Aruch 195:5, Chazon Ovadia (Aveilut v. 1 p. 229), Rav Mordechai Eliyahu's comment on Kitzur Shulchan Aruch 195:3
- Birkei Yosef 340:12, Chazon Ovadia (Aveilut v. 1 p. 229)
- Shulchan Aruch rules that one doesn't have to tear kriyah on an outer garment. The Taz 340:5 explains that this outer garment is something only worn for modesty in a shul or outdoors but not in a private house. The Nekudat Hakesef argues that the outer garment is only exempt if it is something worn for modesty just in shul but if it is a garment always worn in the street even if it isn't worn at home it is still obligated in kriyah. Kitzur Shulchan Aruch 195:3 rules like the Shach and writes that one who is mourning a relative other than a parent doesn't rip one's outer garment which one sometimes goes in the street with and sometimes not. Rav Mordechai Eliyahu's comment on Kitzur Shulchan Aruch 195:4 writes that such a mourner should rip his shirt and not his coat. Rabbi Mordechai Willig (Aveilut Shiur 20) explained that since people don't always wears a jacket inside or even in the street it is considered an outer garment that doesn't need to be ripped.
- Kitzur S”A 195:3 writes that one who is mourning a parent should rip one's outer garment as well as all inner garments until one reveals one's heart, but one doesn't need to rip a coat which sometimes wears. Rav Mordechai Eliyahu's comment on Kitzur S”A 195:4 writes that such a mourner should rip his shirt and not his coat.
- Kitzur S”A 195:3
- Shulchan Aruch YD 340:3, Kitzur Shulchan Aruch 195:3
- Shulchan Aruch YD 340:3, Kitzur Shulchan Aruch 195:3
- Shulchan Aruch YD 340:3
- Kitzur Shulchan Aruch 195:1, Chazon Ovadia (Aveilut v. 1 p. 226)
- Chazon Ovadia (Aveilut v. 1 p. 226) writes that the Sephardic minhag to tear kriyah after the burial has what to rely upon. Rav Mordechai Eliyahu's comment on Kitzur S”A 195:2 confirms this minhag.
- Rambam (Aveilut 11:1), Shulchan Aruch YD 399:13
- Shulchan Aruch YD 401:2
- Kitzur Shulchan Aruch 195:3
- Rav Mordechai Eliyahu comments on Kitzur Shulchan Aruch 195:5
- Moed Katan 22b explicitly states that women have to tear kriyah but for a parent she should tear her inner garment and then turn it and then tear the outer garment. Chazon Ovadia (Aveilut v. 1 p. 236) writes that any rabbi who tells women that they don't need to tear kriyah is mistaken and need to be rebuked harshly.
- Shulchan Aruch YD 340:1, 374:4
- Shulchan Aruch and Rama YD 340:4
- Shulchan Aruch YD 340:6
- Rama YD 340:6
- Shulchan Aruch and Rama YD 340:7
- Shach YD 340:17, Yalkut Yosef YD 9:15
- Shulchan Aruch YD 340:8
- Rama YD 340:8
- The Bet Yosef Y.D 340:6 notes that the Ramban and the Rosh cite the Raavad that this includes from the time of death to the time of burial (also Ritva Moed Katan 25a, see Prishah on the Tur note 10). However, the Rama Y.D 340:5 citing the Raavya limits it to the simple interpretation of being present at the time of death, and the practice is to be lenient (Rambam Avel 9:11 seems to hold this way as well).
- The gemara in Moed Katan 25a praises one who mourns an “adam kasher” and states that in fact it is an obligation. Similarly, the gemara says that if a person was standing by a Jew at the time of death then again there is a requirement to mourn and tear keriya. Tur Y.D 340 cites R Yonah that so long as the person is not suspected of sinning, one rends his garment. He further cites the Maharam Rutenburg who maintains that the enactment includes Jews who at times violate commandments for pleasure or out of laziness, but not apostate Jews. The Shulchan Aruch Y.D 340:5 writes in accordance with R' Yonah, whereas the Rama writes in accordance with the Maharam Rutenburg. The Rambam (Avel 9:11) seems to understand that the gemara was not relating to the previous statement when it cited the law that one rend one’s garment over the passing of another Jew, and thus rules that this applies to all Jews. The Nishmas Avraham Vol 1 (O.C 223 note 6) write that for this enactment to apply, the deceased must be one who is not suspect on any transgressions. See there for further discussion.
The gemara explains that the reason for this enactment is because when a Jew dies it is like a sefer Torah was burned. There is a version of Rashi which elaborates that this line means that the person could have learned more Torah. Some explain that according to Rashi this enactment does not apply to women (Ramban, Ritva, Tur). However, others explain that even Rashi would agree that this enactment applies to women (Rashba, Beit Yosef).
- There are three reasons brought down for the leniency today: 1) the takanah is waived so that people will stand there at the time of death (Shulchan Gavoha YD 339:4, Gesher Hachayim 14:9); 2) the sick person waives the takanah so that people will be there at the time of death (Shut Torat Chesed OC 17:6); 3) practically we cannot allow so much garment rending; otherwise people will run out of clothing and die of cold (Shulchan Gavohah YD 339:4 based on Rama YD 340:7; also Kaf Hachayim OC 547:25). The Nishmas Avraham (vol 2 YD 340:2) holds that there should be a chiyuv as these reasons are weak and the rabbis never worried about such things. However from what he writes in OC 223:6 it is clear that he does not believe that one who does not rend one’s garments is mistaken, nor is it clear that he would advise one to rend, based on common practice not to do so.
Rav Schachter (oral communication cited in Halachos for Professionals) said regarding similar laws of kriya on non-relatives, like a rebbe muvhak, that one should only observe these laws nowadays if his actions will not seem out of place or disproportionate to his relationship with the person or otherwise inappropriate; however if it would not seem out of place or cause stress to the mourners then such expressions would be appropriate.
- The Nishmas Avraham discusses poskim who are lenient not to tear since the coat that they wear on top is not their own (Tzitz Eliezer vol 13 35:4). He himself holds on principle that one would be required to rend the garment underneath for two reasons: first, because some say that if one rends the lower garment one still fulfills his obligation, and second as the white doctors’ coat does not count as an upper garment, rather the lower garment (the doctor’s personal shirt) would count as the upper garment (Rabbi Neuwirth, Rabbi Shlomo Zalman Auerbach). It would seem that today even the Nishmas Avraham would agree not to rend for doctors and nurses who wear scrubs and their lower garments are also not their own. (See Shulchan Aruch YD 340:34)
- Igros Moshe CM vol 2 73:10