Mitzvah of Tefillin

(3) A minor who knows to guard tefillin in purity, such that he will not sleep or flatulate while wearing them -Rema: "and not to enter the bathroom with them" [based on Rashi in the chapter of "The Stolen Lulav"] - his father is obligated to acquire tefillin for him in order to train him [in the commandment]. Rema: "And some say that this 'minor' is only if he is 13 years and one day (Ba’al HaIitur). And so is the custom and one should not change from it.

(3) Women and slaves are exempt from Tefillin because it is a positive commandment which is caused by time. Rem"a: And if the women want to be stringent for themselves, we protest against it (Kol Bo).

(42) The "ש" of the tefillin, it is a law passed down to Moshe from Sinai that he should make in the leather of the compartments [of the tefillin] of the head like the letter "ש" extruding from the creases of the leather, one on its right [side] and one on its left. On the right side of the wearer (ie. the right side of the tefillin box from the perspective of the one wearing it), one places [a "ש"] of three "heads", and on the left side of the wearer, one places [a "ש"] of four "heads". Rema: However, if they were switched it does not invalidate. (Mordechi and SMa"G and Tosefot - Perek Hakometz)

(5) When putting them on, have in mind that God commanded us to put these four passages which contain [the principle of] monotheism and the Exodus on the arm opposite the heart and the head opposite the brain so that we may remember the miracles and wonders that He did for us which indicate His Unity and that He is omnipotent in heaven and on earth to do with them as He wills. And to submit to God his soul, which resides in the brain as well as his heart which represents the main physical desires and thoughts. Through this he will remember the Creator and moderate his pleasure[-seeking]. He should put on the arm-tefillin first and say the blessing "... to place the tefillin" and afterwards put on the head tefillin without a second blessing for only one blessing [is made] for both of them. Note: Some say to make [the following] blessing on the head tefillin "...on the mitzvah of tefillin" even if one did not make an interruption between them (ie. the arm and head tefillin) [The Ro"sh in the Laws of Tefillin]. [And this is the widespread Ashkenazic custom, to make 2 blessings]. And it is a good idea to always say after the second blessing "Blessed is the Name of His glorious Kingdom forever and ever" [Mahar"i ben Chabib Agur, Siman 38]