The Rebbe's guidance on repairing particular wrongs — what a tikkun (rectification) involves:
Repair in the same area. A true tikkun centers on the very area where one went wrong: for Shabbos, a firm commitment to careful observance and repeated study of its practical laws (IK 2435, 5924, 4207); for lapses in prayer, regret, resolve and learning the laws of prayer (IK 9262); for neglecting tefillin, returning at once to daily tefillin (IK 7180); for eating what one shouldn't have, care in that very area (IK 6021); in family purity, sincere teshuvah and renewed observance (IK 6036, 10664, 10938). Rectification comes through strengthening one's intent in mitzvos and adding Torah study (IK 5857).
The general tikkunim. The Rebbe's usual steps were: more Torah study — spoken aloud, and by heart — heartfelt prayer, tzedakah before prayer, and helping others, especially young people, come closer to Torah (IK 1083, 2718, 3398). Learning Iggeres HaTeshuvah in the Tanya is recommended (IK 4625, 814). For worries over possible past errors in mitzvos, increase tzedakah (IK 3005). One may spend beyond a fifth of one's means for spiritual healing, just as one would for bodily health (IK 5791).
Don't dwell on it. With some struggles, the best way is not to think about them at all, even about their rectification, and to turn one's mind to Torah and positive action instead (IK 3986). Blemishes from the past are repaired above all by joy and not brooding over faults (IK 814).
Serious matters. Violence can never be justified, even when one is convinced one is right; it calls for sincere teshuvah (IK 3582). Public transgression must never be treated lightly (IK 7040). After an incident with a Sefer Torah, those involved were given specific fasts and greater care (IK 1950).
See also: Teshuvah – How to do teshuvah; Teshuvah – Asking and granting forgiveness.