Making sure that the Kesher of the Yud of your Tefillin Shel Yad remains Kosher:
It is a Halacha from Moshe on Sinai that a knot in the shape of the letter Yud be formed by the Tefillin Shel Yad after the straps have been entered into its Mabarta. This is done in order to complete G-d’s name of Sha-daiy, of which the letter Daled and Shin are found by the Tefillin Shel Rosh.
Being careful that the Yud does not turn over: [The area of the knot which appears like a Yud has to be made in a way that it appears like a Yud from the outside, meaning, that it appear like a scriptural Yud to those looking at it.] Hence, one needs to be careful by the knot of the Yud that it does not turn over.
Which part is considered the Yud, and which direction should the protruding strap face: It is only the very most top part of the knot that is considered to form the Yud. The part of the strap that protrudes from the top of the Yud is viewed as the bottom of the Yud, while its opposite part, which is the main knot, is considered its top. For this reason, the widespread custom is for the Yud to always be made with the protruding strap facing towards the right, weather by the Tefillin of a righty or a lefty, in order so the letter is written in its proper form, and does not appear upside down. Thus, while by a righty’s Tefillin the protruding strap faces towards the heart away from the Bayis, by a lefty’s Tefillin the protruding strap faces towards the actual Bayis. However, some have a tradition even by a lefty’s Tefillin to make the protruding strap face towards the heart, even though it makes the letter Yud appear upside down.
The protruding strap that comes out of the Yud:
Cutting the protruding strap of the Yud into a slant: It is customary to cut the protruding strap that comes out of the Yud in a slanted position, in order so it appear like a Yud as well.
The length of the protrusion: One must be careful that the strap which protrudes from the Yud is not too long, hence making it look like a Vav, or too short, as it is an integral part of the letter Yud of the knot. It is to extend a minimum measurement of one centimeter. It is to extend one centimeter throughout its entire width, until the start area of the slanted cut.
 Admur 32:79; Michaber 32:52; Menachos 35b; See Piskeiy Teshuvos 32:79-80; See Reshimos 34
 Admur 27:17; Rama 27:10; Mordechai p.96
 Admur and Rama ibid write this Halacha in continuation of the Halacha regarding that the Daled of the Shel Rosh must face towards the outside, and then conclude that the same applies to the Yud of the Shel Yad.
 See Chazon Ish 4:1; Pnei Meivin 1:16; Maharshag 1:32; Piskeiy Teshuvos 27:4
Other opinions: Some Poskim rule on the contrary that the protruding strap is considered the top of the Yud, while the knot itself is considered the bottom. [See Shevet Halevi 5:3] Other Poskim rule that the protruding strap is the entire Yud itself, and is hence to be shaped in the form of a Yud. [Tur 32 in name of Rosh; Ateres Zekeinim 27; Mikdash Me’at 32:198; Orchos Rabbeinu 3:198; Minchas Yitzchak 9:3]
 Devar Halacha 2:122; Tefila Lemoshe 14:1; Piskeiy Teshuvos 27:4
 Orchos Rabbeinu 3:198; Shevet Halevi 5:3
The reason: As according to the Zohar one is to be particularly that the knot of the Yud touches the Bayis.
 See Piskeiy Teshuvos 32:79
 Levush 32:52
 Piskeiy Teshuvos ibid
 Ketzos Hashulchan 1:89 and Shiureiy Torah of Rav A”C Naah printed also by “Mitzvah Lefarseim” that one must be careful in this as is the minimum measurement for all straps of the Tefillin, and he has seen many people who unfortunately are not careful in this and only have it extend a small amount. The Brisker Rav agreed with him in this and stated that it is known one must be careful in this. So also rules Sefer Zeved Tov; However, the Chazon Ish [4:1; Shabbos p. 67] was of the opinion that there is no minimum measurement for this protrusion, and that so is the custom to not be careful that extend 1 centimeter
 See Ketzos Hashulchan ibid in name of Sefer Zeved Tov, and so is implied.