How do I make marble cake out of box cake mix?

make mix

split batter between two bowls

to one bowl add cocoa powder -- at least 2 TBS (Or more for darker) and mix in well.

now pour both into pan at same time, swirling around as you pour.

bake and serve.

same as Doug but I but I pour the yellow cake in first, then the chocolate on top and take a knife and swirl

Doug got it right.