The Rebbe's guidance to a chosson and kallah preparing for marriage:
The spiritual preparation matters most. The material wedding preparations pass; the spiritual foundations last a lifetime, so prepare with modesty and spiritual intent (IK 1627, 2171, 4462). Build the home on Torah and mitzvos, illuminated by Chassidus (IK 6884).
Learn the halachos. Study the relevant halachos with an experienced person before the wedding, together with the chapters of Reishis Chochmah the Rebbe recommended for this time (IK 1339, 2222). Where not everything can be done at once, give priority to the core mitzvos of the home, first of all taharas hamishpacha (IK 10972). See also: Marriage – Family purity (taharas hamishpacha) and the mikveh.
For the chosson: keep learning right up to the wedding without letting up (IK 7205); on beginning to wear Rabbeinu Tam tefillin at the engagement, see IK 632. Marrying a bas Kohen calls for extra diligence in Torah study (IK 1601).
Decisions together: wedding decisions should be made together with both families (IK 7205).
See also: שידוכים – זמן בין אירוסין לחתונה; Marriage – The wedding.