Directives to be followed when sending Tefillin for checking:[1]

• Verify the person is a certified Magiah. [Not every Sofer is certified to also be a Magiah.]
• It is advisable to have the Tefillin checked annually by different Magihim, rather than to use the same one each year.[2]
• Verify that the Magiah will also examine the straps and Batim to verify their Kashrus status.
• If the Sofer states that the Tefillin are Pasul, one should right away put on a Kosher pair of Tefillin that day before sunset. The blessing is to be repeated. This applies even if only the Shel Yad or Shel Rosh were invalid.

Directives to be followed when sending Mezuzos for checking:[3]

• Verify the person is a certified Magiah. [Not every Sofer is certified to also be a Magiah.]
• It is advisable to have the Mezuzos checked annually by different Maghim, rather than to use the same one each year.[4]
• One should place each Mezuzah in an individual bag and label them as to from which room they came. Upon replacing them, one should replace them onto the same doors that they were originally on.[5]
• Any parts of the Mezuzah attachment that one plans to discard, such as an old case, or the tape or glue, is to be placed in Geniza.[6]
• One is to request from the Sofer to make sure that the Shin on the outside of the Mezuzah is not covered over by the parchment when rolled, and that it remains by the center of the Mezuzah when entered into its case.
• If buying new Mezuzah cases, one should use a transparent casing of clear plastic or glass for his Mezuzah parchment. This however is with exception to a room in which marital relations take place, and according to many Poskim also in a room in which people undress, in which case the Mezuzah is not to be visible even through glass or plastic.
• One is permitted to remain the home while the Mezuzos are checked, and he is not required to place up new Mezuzos in the interim.[7] Likewise, he may remove all the Mezuzos of the home, including the front door, for checking at the same time.
• It is best for the husband to put the Mezuzos back up on the doors, rather than the wife, unless he is unable to do so.[8]
• If one replaces the checked Mezuzah up only the next day, a blessing is recited.[9] Likewise, if the Mezuzah was found to be Pasul and then fixed, then a blessing is recited even when put up that same day.[10]

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[1] See Sefer “Inside Stam” by Harav Reuvein Mendelowitz chapters 13

[2] See Inside Stam footnote 600-601

[3] See Sefer “Inside Stam” by Harav Reuvein Mendelowitz chapters 14

[4] See Inside Stam footnote 600-601

[5] See Inside Stam footnote 603

[6] See Admur 42:6; Piskeiy Teshuvos 154:3

[7] See Daas Kedoshim 291:1; Inside Stam footnote 597

[8] See Beir Moshe 2:100

[9] See Aruch Hashulchan 289:4; Nesivim Bisidei Hashlichus Raskin  p. 95

[10] See Mikdash Me’at 288:5