10:2 The time to put on tefillin in the morning is when it becomes light enough to recognize an acquaintance from about six feet away. Tefillin are put on after the tallis. This is because tefillin are only worn on weekdays, while the tallis is also worn on Shabbos and yom tov. Since the tallis is worn more frequently, the order is subject to the rule that more the common mitzvos in a sequence are performed first. However, if one happened to pick up his tefillin first, he should then put them on; since the mitzvah has presented itself, it is unseemly to let it pass by unperformed. (On the other hand, if he put them down and picked up the tallis, he does not then put on the tefillin; see Mishnah Brurah 25:5.)
10:3 Exodus 13:16 tells us that tefillin should be placed “on your hand.” The word “yadcha” (your hand) is written with a seemingly-superfluous letter hei, suggesting “the weak hand” in Hebrew. Therefore, a person places tefillin on his subordinate arm, which for most people is the left. (Wearing tefillin on one’s strong arm does not fulfill the mitzvah even after the fact – see Mishnah Brurah 27:1.) The tefillin are placed on the bicep, adjacent to the heart, tilted slightly towards the torso. The Torah also tells us (Deuteronomy 6:8) that the tefillin of the head are to be worn “between your eyes.” The Oral Law clarifies that this means between the eyes on the x-axis (i.e., horizontally) but at the hairline on the y-axis (vertically). The lower edge of one’s tefillin should be no lower than the hairline and the upper edge should be no higher than the place where a baby’s head is soft. One should take care to ensure that his tefillin are properly placed since being even a little out of bounds prevents the mitzvah from being fulfilled and renders the blessing uttered in vain. The knot in the strap of the head tefillin is centered on the back of the head at the base of the skull. One should strive to ensure that his head tefillin are properly tightened.