Multiple changing lines

I'm sure this is in here somewhere but i've been trying to figure out what is the most traditional approach to interpret multiple lines.
I recently got Hexagram 25.1.3.4.5.6 > 15.
Iv'e read about reading only the unchanging line of the first Hex, or only the unchanging line of the resulting hexagram.
Not sure if this is only for cases of conflicting lines though.
Also, I would like to know if either of these approaches is traditional or more recent adaptations.
I really want to read this one correctly.
Thanks!