Techniques: Cable Cast-On
For the first stitch only (used when you have no stitches on the needles):
Starting with a slip-knot on the left needle, insert the tip of the right needle into the slip-knot as if to knit, and knit one stitch. Slip the knitted stitch knitwise from the right-hand needle to the left-hand needle. You will now have two stitches on the left-hand needle.
For remaining stitches, or when you are casting on when there are already at least 2 stitches on the needle:
Insert right needle between the first and second stitches on the left needle (below the needle and above the existing work). Draw up a loop, and place it on the left needle. Repeat for as many stitches as are needed.
Please note that the only time you should be knitting into an existing stitch is when you only have one stitch (or slip-knot) on the left-hand needle. Normally you should be inserting your right needle in between two stitches.