You first need to decide if you want to use HDMI cables. If the plasma has an HDMI input and the cable box has and HDMI output, then all you need to do is connect the HDMI out from the cable box to the receiver and the HDMI out from the receiver to the plasma - then configure the receiver (see the manual - Onkyo manuals are well written and fairly easy to follow).
If the cable box doesn't support HDMI, you will have to use component video cables for the video and optical or coaxial cables for the digital audio. You could still use HDMI from the receiver to plasma because the 674 can upconvert component video to HDMI.