Cable cast-on

Make a slip knot with a short tail on one needle and hold this in your left hand (if you are right-handed).
Insert the other needle as if to knit, wrap the yarn from the ball counterclockwise around the tip, and pull the new stitch through the first loop.

Slip the new stitch onto the left needle in the direction shown. Two stitches are now on the left needle.

Now insert the right needle between the two previous stitches (not through the stitch loop), wrap the yarn as before, and pull through to make another stitch.

Slip the new stitch onto the left needle as step 2. Repeat from step 3 as many times as required.