יהיה הקלף מעור בהמה וחיה ועוף הטהורים - The klaf should be made from the hide of kosher domesticate animals, undomesticated animals and birds:
The Gemara Shabbos (108a) derives from the pasuk (Shemos 13:9), למען תהיה תורת ה' בפיך — In order that the Torah of Hashem should be in your mouth – that the klaf in one’s tefillin must be made from that which is permitted for one to eat.
Primarily this means that the hide that is used for one’s tefillin must come from a kosher creature rather than from a non-kosher creature. Therefore, one may take the hide of a kosher domesticated animal, an undomesticated animal or even a bird. The Gemara asks whether one may use the skin of a kosher fish and since the Gemara does not definitively resolve that inquiry Poskim adopt a stringent position that one may not use the skin of even a kosher fish for one’s tefillin.
The requirement that one use something that one is permitted to eat does not mean that the hide must be taken from a properly slaughtered animal or bird. A Beraisa (Shabbos 108a) teaches that the emphasis of the drosha/interpretation is that the hide must come from a kosher species of animal but the particular animal that is used does not have to be permitted for consumption. Consequently, it is permitted to take the hide of a neveilah or tereifah animal to manufacture klaf.
Bais Yosef mentions (See Biur Halacha הקלף יהיה ה"ד) that preferably one should take the hide of an animal fetus. Sefer HaTerumah (ז"ר' סי) offers a simple reason for this choice which is that the skin is relatively soft and pliable making it easier to manufacture. Additionally, one can be certain the animal had not yet been used for sin. Rama MiPano explains (שו"ת רמ"ע מפאנו סי' ל"ז) that since the skin of a fetus never entered the world alive it never benefitted from the physical world and as such is not subject to Divine justice.
The hole must be small enough that when the kulmus (quill) passes over the hole the hole becomes filled with דיו (ink) but if the hole is larger causing the letter to appear split in two it is invalid. Even if the hole is in the middle of the thickness of a letter and is completely surrounded by דיו it is invalid. (M.B. 32)
If after the letter was written properly, the klaf was punctured its validity depends upon whether the letter retains its essential shape as will be explained in seif 16. (M.B. 32)