This summary is drawn from the Rebbe's letters to him (about 5719–5721, 1959–1961); I did not find a published biography.
Outreach. The Rebbe encouraged ongoing efforts to unite Jews and spread Chassidus, especially around Yud-Tes Kislev (IK 7088), taught that mitzvah campaigns such as distributing shmurah matzah have a profound impact (IK 7317), and supported expanding Chabad's reach, including offering suitable incentives to students (IK 8024).
Education and study. Involvement in pure Jewish education (chinuch al taharas hakodesh) should never come at the expense of one's own spiritual obligations (IK 7292). Learning Ein Yaakov between Mincha and Maariv is especially fitting (IK 7414).
Practical guidance. Guests should always be welcomed warmly, and improvements made with kindness (IK 6970). Before making halachic decisions about electricity on Shabbos in a new setting, consult qualified authorities (IK 6792).
Sources: the Rebbe's letters cited above.