You would do exactly what you've done for the front and rear speakers. You'd split the signal into two - one would go to the center channel, and one would go to the sub.
Unless you know which cable is for which speaker, I'd suggest connecting one of them to the center input first, play some dialog at a fairly quiet volume, and see if you hear voices. If not, then plug that cable into the sub input and plug the other cable into the center input.
