This is the second book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of the Sefer Torah, Tefillin, and Mezuzah.
This chapter explains that there are eight laws given to Moses on Mount Sinai that are fundamental to Tefillin. These include the shape of the head Tefillin and how the scrolls are arranged inside, which one goes on the right and which on the left. It also covers whether the arm Tefillin is written on one scroll and what the law is if it is written on four. Finally, it explains how the scrolls are rolled and whether the leather covering the Tefillin needs to be tanned for their sanctity. Finally, it describes how they are sewn, how the strips are made, and if a strip is cut.