That is that He commanded us to put on the head tefillin. And that is His saying, "and they shall be for symbols between your eyes" (Deuteronomy 6:8). And this command has already been repeated four times. (See Parashat Bo, Vaetchanan and Ekev; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 1.)

That is that He commanded us to put on the hand tefillin. And that is His saying, "And you shall tie them as a sign upon your hand" (Deuteronomy 6:8). And this command was already repeated four times. And the proof that the head tefillin and the hand tefillin are two commandments is their saying in the Gemara (Menachot 44a) in surprise at one who thinks that one not put on the head tefillin or the hand tefillin without the other, but rather only if they are both together - and this is the language of the statement - "one who does not have two commandments, should he also not do the one commandment?" This means to say, should one who cannot do two commandments not do the one - that is not the case, but rather he should do the one commandment that presents itself. Hence he should put on either one of them that presents itself to him. Behold it has been made clear to you that they called the hand tefillin and the head tefillin, two commandments. And these two commandments are not obligatory for women on account of His, may He be exalted, saying, as a reason for their obligation, "in order that the Torah of the Lord may be in your mouth" (Exodus 13:9) - and women are not obligated in Torah study. And so did they explain in the Mekhilta. And likewise did they explain all the regulations of these two commandments in the fourth chapter of Menachot. (See Parashat Bo, Vaetchanan and Ekev; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 1.)

§ The mishna teaches: Absence of the phylacteries of the arm does not prevent fulfillment of the mitzva of the phylacteries of the head, and absence of the phylacteries of the head does not prevent fulfillment of the mitzva of the phylacteries of the arm. Rav Ḥisda said: They taught this only in a case where one has the other phylacteries, but they are not with him or he is unable to wear them for some reason. But if he does not have the other phylacteries at all, then their absence does prevent the fulfillment of the mitzva to don the phylacteries that he has.

Later on, the students said to him: Do you still say that? Rav Ḥisda said to them: No, rather I would say the opposite: Concerning one who does not have the ability to fulfill two mitzvot, should he also not perform the one mitzva that he does have the ability to fulfill? The Gemara asks: And what did he hold initially when he said not to don one of the phylacteries in the absence of the other? The Gemara answers: He held that it was due to a rabbinic decree, lest he be negligent and not try to acquire the phylacteries that he lacks.

Four passages [of the Torah]: Kadesh Li and V'hayah ki y'viacha Ado-nai in the book of Exodus 13:1-10 and 13:11-16) and Shema and V'hayah im shamo'a (Deuteronomy 6:4-9) and 11:13-21) should be written separately and covered with leather. They are called tefillin.
They are placed on the head and tied on the arm. According to Torah law, even a mere point of one of the letters from these four passages prevents all of them from being acceptable. All four must be written in the proper manner.

[Torah] scrolls, tefillin, and mezuzot may not be written on hide from a non-kosher animal, fowl, or wild animal. One may write on the hides of [all] kosher animals, wild beasts, and fowl. This applies even when these animals died without being ritually slaughtered or when they were killed by wild beasts.
We may not write on the skin of a kosher fish because of the foul secretions, since the processing of the skin will not cause the foul secretions to cease.

It is a halachah transmitted to Moses on Mount Sinai that a Torah scroll or mezuzah should be written only [on parchment] which has been ruled. [The parchment used for] tefillin, however, need not be ruled, because they are covered.
It is permissible to write tefillin and mezuzot without [looking at] an existent text, because everyone is familiar with these passages. It is, however, forbidden to write even one letter of a Torah scroll without [looking at] an existent text.

In what manner are the tefillin [placed on] the head written? [The] four passages are written on four parchments and rolled closed, each as a separate entity. They are placed in four compartments, which are covered by a single piece of leather.
The four passages of [the tefillin placed on] the arm are written on four columns on a single parchment. They should be rolled closed like a Torah scroll from the end to the beginning and placed in a single compartment.

After a person writes tefillin himself, purchases them from an expert, or purchases them from another person and has them inspected, and places them in their leather [compartments], they need not ever be checked again.
As long as their compartments are intact, they are assumed to be acceptable even though several years have passed. We do not suspect that a letter has faded out or been perforated. Hillel the elder stated: "These [tefillin] are from my maternal grandfather."

There are eight requirements in the making of tefillin. All of them are halachot transmitted to Moses on Mount Sinai and, therefore, it is necessary to fulfill them all. If one deviates with regard to any of them, the [tefillin] are unacceptable. They are:
a) The tefillin must be square and must be sewn closed in a square. [Both] diagonals must be equal, and thus all four angles will be equal.
b) The leather of the head [tefillin] should have a shin embossed on both its right and left sides.
c) The passages should be wrapped in fabric.
d) A hair should be wound around that fabric. Afterwards, they should be placed in their compartment.
e) They should be sewn [closed] with the sinews [of an animal].
f) The leather compartment in which they are placed should have a place for the straps to pass through so that they can be moved through the [tefillin's] handle.
g) The straps should be black.
h) The knot with which they are tied should be the renowned knot that is formed like a dalet.

What is the order of the passages? For the head tefillah, the final passage, V'hayah im shamo'a, is placed in the first compartment on the right side of the person putting on the tefillin. Shema is placed next to it. V'hayah ki y'viacha is placed in the third compartment next to Shema, and Kadesh Li is placed in the fourth compartment, on the left side of the person putting on the tefillin.
Thus, a person who is facing the person wearing the tefillin will read them in the following order. If their order is altered, they are not acceptable.

[The passages for] the arm tefillin are written on four columns on a single piece of parchment like a Torah scroll, according to the order in which these passages are found in the Torah, in the following manner:
If they were written on four separate pieces of parchment and placed in the same compartment, one fulfills one's obligation. There is no need to glue them together.

The outer surface of the straps of both the head and the arm tefillin must be black. This is a halachah transmitted to Moses on Mount Sinai.
In contrast, with regard to the inner surface, since it faces the inside, it is acceptable if it is green or white. One should not make this [side of the straps] red, since it will be embarrassing for him if they become overturned.
The back of the straps should be the same color as the compartment; if it is green, they should be green; if it is white, they should be white. It is attractive for tefillin to be entirely black, the compartments and the entire strap.

Where are the head tefillin placed? They should be placed at the point of the skull, the end of the hairline towards the face, the place where a child's brain [can be felt] to pulsate.
Care must be taken to position them in the center, so that they will be "between the eyes." The knot should be at the top portion of the neck, the bottom of the skull.

The arm [tefillin] should be tied to one's left arm at the muscle - i.e., the bulging flesh of the arm between the shoulder and the elbow. Thus, if one presses his arm to his ribs, the tefillah will be opposite his heart, thus fulfilling the directive [Deuteronomy 6:6], "And these words... shall be upon your heart."