A neat observation: Do you know those tassle-like things hanging down from the edges of the talis? You will notice that there are none near the corner, right? The reason for that is that it removes any doubts about the measuring of the distance to the hole. Although those tassles are not of a full weave, they may, in some way, be considered an extension of the garment, calling into question from where to measure the actual edge.
Here is a major topic: the number of strings in a tzitzis-fringe! The Torah uses both the word ‘g’dilim’ (lit. twisted threads – see Devarim 22:12) and p’sil’ (thread – see Bamidbar 15:38) in connection to tzitzis. Without going into the full derivation, the Oral Tradition instructs us, based upon those two words, that there should be four strings (originally two white and two dyed with techeiles), which are doubled to eight after they are threaded through the hole. Most opinions rule that to place more than eight is forbidden as per the prohibition of adding to the mitzvos. All agree that to place less than eight is not valid. If one is cutting the strings from a spool, it is crucial that the strings be cut before the minimum number of knots is tied (see below); and preferably before they are even threaded through the hole. Sources teach that it is preferable not to cut the strings with a metal instrument. (Some use their teeth!)
How are the knots tied and the ‘shamash’ (i.e. long “servant” string) string wrapped around the others? We will answer that first according to the bare minimum required by the Torah, and then according to the common custom. (Note: This is per corner)
Torah: There must be one double knot, followed by one group of at least three winds and one more double-knot.
Custom: Five double knots in total, with four groups of winds between the five knots. Beginning with the first group of winds (i.e. closest to the garment), these groups should have seven, eight eleven and thirteen winds respectively. (Explanation: Seven and eight make 15, which is the numerical value of ‘yud’ and ‘hey’, which spell Hashem’s two-letter Name. Adding eleven makes twenty-six, which is the numerical value of the four-letter Name - i.e. adding ‘vav’ and ‘hey’. Thirteen is the numerical value of the word ‘Echad (One)’. Together the winds represent ‘Hashem Echad’ (“Hashem is One”). In addition, the five double-knots represent the five Chumashim (Books of the Torah), with the full number of ten representing the ten ‘sefiros’ (emanations/attributes) of Hashem in the world. Ideally, the knots should be equidistant, so that the more winds, the closer together each wind is to its neighbor. Also, remember that the entire ‘g’dil’ (knotted and braided) section of the fringe should preferably be one-half of the length of the ‘p’sil’ (loose string) section.
The stings should be not too thin and not too thick, as this beautifies the mitzvah.
We have also mentioned that the first knot on the fringe (from the garment) should be pulled tightly (i.e. making a small loop) causing the fringe to hang permanently over the side of the talis, not hanging straight down. At the very least, the fringe should not hang diagonally over the tip of the corner, as that was the practice of those sects who rejected the Oral Tradition!
Finally, it is permissible to attach reinforcing material onto the corner and thread the tzitzis fringe through it as well, even if it is made of leather. As we have learned, it is secondary to the primary material of the garment, and is thus not a barrier to the tzitzis.