Cf. Siddur, p. 12
The following is a general directive. One begins to put on tefillin two months before one’s bar-mitzvah,1 though one begins to recite the blessing only after a few weeks have passed.2
It is the custom of some people to put on the hand-tefillin while seated, for reasons stated in the Zohar, and to be seated while reciting the [preceding] blessing.3
After placing the hand-tefillin on the biceps, but before tightening it in place, one recites the blessing להניח תפילין, bearing in mind as well the head-tefillin. The slip-knot is then drawn tight in order to fasten the thong [that passes through the base of the tefillin (Fig. 12) before encircling the arm]. (One must be careful that the yud-shaped knot at the end of the thong should not slip loose from the cube of the tefillin.) Some people then proceed to bind the thong twice over the base of the tefillin and around the arm, so that together with the initial circuit of the thong [that began by passing through the base of the tefillin] the letter ש is formed.4

[The Rebbe Rashab wrote:] “With regard to the thong that is wound around the arm and hand, my father (the Rebbe Maharash) taught me to wind it seven times, including the two half-circuits [at either end of the forearm. Counting from the crease of the elbow, the pattern is thus as follows (Fig. 13):] The first half-circuit is followed by two full circuits close to each other; a space is then left, followed by four complete circuits, and then the final diagonal half-circuit which crosses the back of the hand. There, together with an additional circuit that is now wound around the hand, it forms something like the letter ד in reverse.”5

“I am well nigh certain that the custom as I saw it practiced by my father (the Rebbe Maharash) was the following: From the palm of the hand, the thong is wound once around the base joint of the middle finger, once around the middle joint, and once again on top of the first circuit.6 The remainder of the thong is then wound repeatedly around the hand on top of the additional circuit that was earlier wound around the palm of the hand. (This differs from the custom of those who make the remaining circuits around the hand radiate fanwise, so that they form the letter ש on the back of the hand.)”7
It was observed that in the course of the davenen the Rebbe Rashab would see to it that the third circuit around the middle finger should not completely cover the first circuit; rather, it was a little closer to the second circuit, where it was held in place by the edge of the first.8
One should take particular care that the head-tefillin is constantly positioned exactly at the midpoint of the width of the head.9
If one speaks between putting on the hand-tefillin and the head-tefillin, the blessing על מצות תפילין should be said over the latter.37
The two side thongs hanging from the head-tefillin should extend to the legs.10
It has become customary to use another’s tefillin [when necessary,] even though this may mean that the knot of the head-tefillin may have to be changed to fit. This does not affect the knot’s being considered permanent.11
Those who pray in four pairs of tefillin should follow this order: (1) The hand-tefillin and head-tefillin of Rashi are put on before איזהו מקומן (Siddur, p. 23), and one prays in them until after אך צדיקים (p. 85). (2) The head-tefillin of Rashi is removed, and replaced — without a blessing — by that of Shimusha Rabba. The Shema is recited until Emes, and Tehillim is recited according to the day of the month. It was the custom of those who were particular, to study the commentaries of Rashi and Metzudos on the daily reading of Tehillim. (3) One [removes these tefillin,] puts on the tefillin of Rabbeinu Tam, again without a blessing, and recites the Shema (p. 46) until Emes, the passage beginning קדש (p. 85), and the שש זכירות as they appear in the Siddur (p. 86). One then studies a chapter of Mishnayos according to his level of understanding. (4) The head-tefillin of Rabbeinu Tam is removed, and replaced — without a blessing — by that of Raavad. One recites the Shema until Emes, and studies the portion of the week’s Sidra (with the commentary of Rashi) that corresponds to the day of the week; e.g., on Sunday until Sheni, on Monday until Shlishi, and so on.12