The information in this tutorial is based on the scarf that I am currently knitting, as that seemed the best way to detail everything, but the concept is obviously universal.
For my current project, rows 1/3/7 are basic knit and rows 2/4/6/8 are purl. Each row is 24 stitches.
The fifth row is the cable row. Here is how it works:
Knit the first 6 stitches.
Slide the next 6 stitches onto the cable needle and hold in back. The "cable needle" doesn't actually have to be anything special - I am using a dp needle of the same size as the needles I am using to knit the scarf.
Knit the next 6 stitches (stitches 13-18 on the main needle). The cable needle should remain in back.
You will now have 12 stitches knit for this row, 6 remaining on the cable needle, and 6 remaining on the other needle.
Now you want to knit the 6 stitches that are on the cable needle.
When you are done with this step, you won't need your cable needle again until you reach your next cable row.
Last, but certainly not least, you knit the final 6 stitches.
Repeat those 8 rows, and the cable pattern emerges!
I believe (Hannah please correct me if I am wrong) that some cable patterns will call for the cable needle to be held in front, instead of the back, but I'm sure it's all spelled out in the pattern.
Since I last posted about this scarf, I've come a long way. Not far enough to call the scarf done for Jill's birthday tomorrow, but close enough that I can probably convince her that I won't extend her birthday too long. I don't have too much length to add to the scarf, but I need to add the icord edging. The scarf actually did a lot of twisting in on itself, so I'm hoping the edging will minimize that (I also some heavy books to hold down the edges so they would drive me a little less crazy).