The [fringe] strings have to be spun for their purpose.Rem"a: Some authorities are stringent to even comb them for their purpose. And the custom is to be lenient regarding combing. (Mordechai; Agudah, Section 23)[The spinner] should say at the start of the spinning that he is doing it for the purpose of fringes. Or he should say to a woman [who doing the spinning] "spin me fringes for a shawl." And if the strings were not spun for their purpose, they are unfit.
If they are spun by a non-Jew and a Jew stands behind him and tells him to make the strings for the purpose [of fringes], they are unacceptable according to the Rambam and acceptable according to the Rosh. Rem"a: And it is our custom that the Jew should do a tiny bit of work as stated in Chapter 22, Paragraph 9 and in Yoreh Deah, Chapter 171 on tefillin and Torah scrolls, respectively. And the interweaving must also be done for the purpose.
If the [ציצית strings] unraveled from their interwoven [state] and became sixteen strings (ie. each of the 8 ציצית strings was originally made of 2 interwoven strings), it is valid. And this is only if there remains [a section] from the interwoven string that's long enough to tie a bow. Rem"a: And ideally, it is good to tie the strings at the bottom, as is seen below in Se'if 14 in this [same] Siman [Beit Yosef in the name of the Rokeach].
The Length of the 8 strings should not be less than 4 thumb-widths, and there are those that say 12 thumb-widths, and that is the custom. And the upper bound has no [specific] measurement. Rema: And if the length was made more than necessary, one is able to cut it and there is no concern for [the tzitzit invalidation of] you should make it and it should not be made (Mordechai BH"K). One of the strings should be longer, in order to wrap the fringe with it (that's that part of the tzitzit that is not combined). Rema: And the aforementioned measurement is for the tzitzit after they are tied, excluding [the part of the tzitzit] that is left [hanging] on the corner of the garment (reasoning of the Beit Yosef).
One should not make tzitzit from wool became attached with thorns when the sheep was lying between them, and not from the hairs that were plucked from the animal, and not from the remnants of the warp that the weaver leaves over at the end of [the making of] the garment, and the reason is because it is an embarrassment to the commandment.
If they were made from stolen wool it is invalid, since it is written and it should be made for them, [meaning] from that which is theirs. Rema: And this is precisely that the strings were stolen, but if the wool was stolen, and it was made into strings, it is valid, however, from the outset, it is forbidden to do that (Beis Yosef in the name of Nimukei Yosef in the laws of tzitzit). And with regards to the topic of making the blessing, see later on at the beginning of chapter 649 (see M"S R"S T"V).
Borrowed strings, they are a loan that the object is not returned, and they are considered his.
One who bows [in worship] to an animal, its wool is invalid for tzitzit. One who bows [in worship] to planted flax, [the linen from the flax] it valid for tzitzit because they were changed.
He makes a hole in the length of the talit not above 3 fingerbreadths which are the thumbs (Kol Bo, chapter 22, and Maymoni and HGH' SM"K chapter 31) because [above that] is not called the corner. And not below the measurement from the joint of the thumb until the end of its fingernail, because it says on the corner and if it were to be below the full joint of the thumb, it would be below the corner. Rema: And this measurement is the straight measurement (perpendicular to the edge) and not the diagonal from the corner (Beis Yosef).
If it was further from the corner [the measurement of the] full knuckle of the thumb, and the fabric was torn from the [place of the] strings until this measurement no longer remained, it is valid, since it had the correct measurement at the time that the tzitzit were placed on it. Rema: And we are accustomed to make a rim around the hole so that it shouldn't tear there and be less than the correct measurement. And so too, we make a rim on the edges of the garment below because of this reasoning (Beis Yosef in the name of the SM"K and Kol Bo). And there are those that say that with the width of the garment there is no correct measurement, and there are those that say that the law of the width of the garment is the same law as the length, and their words (the second opinion) seem correct.
If the corner is what we call an אורילייזא and is wide, Tzitzit shouldn’t be put on it. If one put Tzitzit on the garment, it is invalid since it is written “on the corner of your garments” and this is not considered from the garment. Rather, there must remain at least the measuremt of joint of the thumb and not farther than 3 fingers since the hole must be in the garment. [It is good to measure the joint of the thumb without including the fringes while measuring the 3 fingers including the fringe.]
The number of the Tzitzit threads on all corners is 4 strings folded over, which is 8. If more were added, they are invalid. You should trim the tips of the 4 threads then insert them in the corner. Finally, double them over and then there will be 8.
One should be careful the cut the tips of the threads into 8 before wrapping them. For even if one thread was wrapped and tied with just one tie and afterwords cut, this is invalid by the principle of תעשׂה ולא מן העשׂוי since they they were made defective.
You should take 4 threads on one side and 4 threads on the other side and tie them two times, making a double knot. Afterwards, you should use wrap the longest thread around the 7 others with a little wrapping and then make a double knot. Then repeat and wrap. This should be repeated until 5 double knots are made with the 4 spaces between them filled with windings. There is no שׁיעור (set measurement) for wrapping other than the fact that all the wrapping and the knots must be 4 thumb widths wide and the branch (protruding threads) 8 thumb widths. [If the Tzitzit are longer, one should make sure that one the tassels should be (at the minimum) one third of the length and the branch should be two thirds of the length.] It is the custom to have 7 wrappings in the first space, 9 in the second space, 11 in the third, and 13 in the fourth. They all add up to 40 corresponding to the numerical value of “השׁם אחד” (G-d is one) which is actually 39 and by adding G-d (since G-d is one, you add one), it makes 40. It is customary to tie a knot at the end of every thread so that they should remain intertwined.
There are those who require the Tzitzit to hang by the length of the Talit since they must drop by an angle [he means “hang by an angle”]. If the Tzitzit were placed on the width of the Talit, they would not drop but would hang facing the ground. Some say that no material should be in the holes of the talit containing the Tzitzit. Others allow this and so is our custom.