In the area of Orach Chaim, when there is no established Lubavitch Minhag, i.e. in a case which is not in the Shulchan Aruch Harav (for whatever reason), and is not discussed by any other Lubavitch authorities, which Halacha Sefer (aruch hashulchan, mishna berura etc.) do Chabad-Chassidim tend to follow?
There isn't one specific source for every minhag and it would be easier to answer if you were asking about something specific. Some of the sources for this subject are the Shulchan Aruch Harav, the siddur, Sefer Haminhagim, and Kovetz Minhagim. If you want to know how to do a certain thing, I would recommend speaking with a Chabad rabbi.