Q. How to put on Tefillin?
A. Ashkenazim and Chassidim put on and remove the Tefillin while standing, while Sephardim do so while sitting. It is forbidden to speak or be distracted while putting on the Tefillin.
The arm-Tefillin is laid on the inner side of the bare left arm, above the elbow, so that when the arm is bent the Tefillin faces towards the heart. If one is left-handed, it is placed on the right arm in the same place. The blessing is then said on the the arm-Tefillin it is;
Boruch atah Ado-noy Eloheynu Melech Ha’olom asher kedishanu bemitzvosav vetzivanu lehani’ach Tefillin.
After the Blessing is recited the Tefillin is tightened. It is extremely important that the Tefillin knot be touching the actual Tefillin box (if not it is questionable if you fulfilled the Mitzvah) the strap is then wound seven times around the arm. Then the head-Tefillin is put on, before putting on the head Tefillin the Ashkenazik custom is to recite a second Blessing;
Boruch atah Ado-noy Eloheynu Melech Ha’olom asher kedishanu bemitzvosav vetzivanu al mitzvat Tefillin.
The Sephardic and Chabad custom is that no blessing is said for the head-Tefillin, the first blessing sufficing for both. However if they spoke about something not related to Tefillin they recite the blessing on the Head Tefillin.
After the Blessing on the head Tefillin it is placed on the middle of the head just above the forehead, so that no parts rests below the hairline. The knot of the head-tefillin sits at the back of the head, upon the part of the occipital bone that protrudes just above the nape, directly opposite the optic chiasm. The two straps of the head-tefillin are brought in front of the shoulders, with their blackened side facing outwards.
Now the remainder of the arm-tefillin straps are wound three times around the middle finger and around the hand to form the shape of the Hebrew letter of either a shin (ש) according to Ashkenazim, or a daled (ד) according to Sephardim. There are various customs regarding winding the strap on the arm and hand. In fact, the arm strap is looped for clockwise wrapping with Ashkenazi Tefillin while it is knotted for counter clockwise wrapping with Sephardic and Chabad Tefillin. On removing the Tefillin, the steps are reversed.
Tefillin used to be worn all day, however now days with all the profanity in the streets and with all our modern technology the Rabbis were concerned that we would go places not befitting the Tefillin or get caught up in the moment and forget we are wearing the Tefillin. Therefore, it is customary to only wear Tefillin while praying. One can say just the Shema prayer in a few minutes or recite the entire davening for an hour, there is no time limit to wearing the Tefillin as long as you are conducting yourself appropriately.
If you want to put the Tefillin on again later in the day you may do so. However, if it is more then 3 hours since the Tefillin were removed or within the three hours but you did not have in mind to put them back on when they were taken off or you used the restroom in between, you must make another Blessing.
How to Wear & adjust Tefillin Shel Rosh
How to properly put on Tefillin