First of all, take a look at your pattern. Usually how to make a cable is listed in a definition of stitches somewhere in the pattern.
This is the definition of a C4B:
C4B - Cable 4 back: put next 2 stitches on cable needle at back of work, knit 2 From your left hand needle, knit the 2 from the cable needle.
Here's Amy's video
on cabling. Scroll down to Decorative Stitches.