1. Which part of the animals skin to use as parchment:
Initially it is Mitzvah to write the Mezuzah on the Duchsutis part of the animal’s skin. However Bedieved if it was written on the Klaf or on the Gevil it remains Kosher. Today’s Klaf is kosher according to all.
2. Processing the parchment:
Initially the parchment needs to be processed for the sake of the Mitzvah. However if this type of parchment is not readily available, and will thus cause him to delay doing the Mitzvah if he waits until its availability, then he should write the Mezuzah on unprocessed parchment and do the Mitzvah immediately [without a bracha], until processed parchment becomes available.
3. Rolling the parchment:
The Mezuzah is to be rolled from its end [left] to beginning [right], towards the word Shema.
4. Having the Shin visible:
The custom is to make a hole in the tube opposite the name Shakay, which is written on the outside of the parchment, in order for it to be visible to the outside.
 This is the inner part of the skin after it is split into two.
 The outer part of the skin when split into two
 The entire skin without being split
 Michaber ibid
 Rama ibid; In today’s times [of the Rama] the skin is not split to two but is rather sanded down, and it is much better than Gvil or Duchsustis. [See Rama 271/3; Shach 271/8-9; Aruch Hashulchan 288/2]
 Michaber 288/5
 This follows the opinion of the Rambam which doesn’t require the processing of Mezuzah parchment to be done for the sake of the Mitzvah. Other opinions: According to majority of the Poskim it is invalid even Bedieved if the parchment was not processed for the sake of the Mitzvah. So rules the Bach; and therefore says the Bach it should never be used even if no other parchment is available, [as if it were to be allowed to be used even temporarily, one may not place enough effort in his search for a kosher parchment, and thus it is better for him to leave his door without Mezuzahs. [Taz 288/2] The Shach himself rules that one may rely on Rambam in a case that there is no other way to fulfill the Mitzvah, as explained by Michaber. Nevertheless a blessing should not be said. [Shach 288/4] The Taz 288/2 comes to the same conclusion.
 Shach 288/4; Taz 288/2; See previous note; However from Michaber it is implied that a blessing may be said as he holds it only needs to be processed initially for the sake of the Mitzvah. However from Taz ibid it is evident that he learns that the Michaber himself also holds that a blessing should not be said.
 Michaber 289/14; See also 289/6
 Rama 288/15