The Rebbe's letters of consolation and his guidance for mourners, gathered by the questions people actually ask. Each page summarizes his guidance and cites the letters (IK = Igros Kodesh letter number).
Grief, comfort and honoring the departed — The eternity of the soul and the limits of mourning, turning grief into action for the soul's elevation, finding comfort, and consoling others.
Kaddish, yahrzeit and the tombstone — Kaddish and the year of mourning according to Chabad custom, yahrzeit observance, and erecting and inscribing a matzeivah.
Widows and orphans — Supporting widows and orphans, and the Rebbe's words to orphaned bar mitzvah boys.
In short: The Rebbe taught that the soul is eternal and continues to ascend, so mourning has its proper limits; the truest way to honor someone who has passed away is to turn grief into good deeds — Torah, tzedakah and mitzvos in their merit — which elevate the soul and bring comfort to the mourners. The traditional words of comfort tie every personal loss to the mourning for Zion and to the certain comfort of the redemption. Practical questions of Kaddish, yahrzeit and the tombstone follow Chabad custom and the guidance of a local Rav, and a matzeivah should not be delayed without reason.
See also: Bitachon – Hardship, loss and asking why; Moshiach and Geulah – The era of redemption (resurrection).