שלום וברכה!
[שלהי תשט"ז]
... במש"כ ע"ד התחלה בעבודת התפלה - נכון הדבר במאד. אלא שמתחלה ילמוד איזה פעמים קונטרס התפלה (עכ"פ עניני העבודה אשר בו) ובזמן הראשון ינהג בהנ"ל בתפלת ש"ק.
Shalom u'Vrachah! [End of 5716]
... Regarding what you wrote about beginning avodah in prayer—it is very appropriate. However, at first, you should study Kuntres HaTefillah several times (at least the sections on avodah), and in the initial period, you should practice the above specifically during Shabbos prayers.
Summary
The Rebbe teaches that one should begin avodah in prayer by first studying Kuntres HaTefillah, focusing on its sections about avodah, and initially apply these lessons during Shabbos prayers for best results.