The Cable Cast-On tends to produce a slightly looser cast on edge and can be easier to get a good tension on.

Like the knit cast-on, begin by making a slip knot and popping onto one needle.

Put the other needle through the slip knot (your first stitch) from front to back in exactly the same way as the knit cast on. 

Wrap the yarn around the right needle and pull it back through.  This is your next stitch - hook it back onto the left needle without twisting.

Now, unlike the knit cast-on, put the right needle BETWEEN the first and second stitches from front to back. 

Wrap the yarn around the right needle in the same way as before and pull the loop back through.  This is your next stitch.  Hook it onto the left needle. 

By continuing to cast-on by putting the right needle between the last two stitches made, you are creating a cable cast-on.