שלום וברכה!
[כ"ו מ"ח תשכ"ו]
ובמענה לשאלתו - ילמוד ויהא בקי: א) בהל' תפלין הצריכות (לו, ולא לסופר). ב) באחדים מדרושי דא"ח המדברים בתפלין (בלקו"ת, סידור, אמ"ב וכיו"ב).
מהעתקה.
ובמענה לשאלתו: נכתב לא' שכתב על חסרון בקיום מצות תפלין. וראה לעיל חכ"ג אגרת ח'תרפו. ט'ב, ובהנסמן בהערות שם. לקמן ט'רנט. ט'רסב. ט'ש.
Shalom u'Vrachah!
[26 Menachem Av 5726]
In response to your question — you should study and become proficient: a) in the laws of tefillin that are required for the wearer (not for the scribe). b) In several Chassidic discourses that discuss tefillin (in Likkutei Torah, Siddur, Amud HaTefillah, and similar works). This is from a copy.
And in response to your question: It was written to someone who wrote about a deficiency in fulfilling the mitzvah of tefillin. See above, volume 23, letter 8790. Also see what is noted there in the footnotes. See also below letters 6599, 7622, and 8000.
Summary
The Rebbe advises diligent study of both the practical laws of tefillin for those who wear them and related Chassidic teachings, emphasizing thorough knowledge and referencing prior correspondence on improving tefillin observance.