After you have finished the cast on and the initial four rows, you are ready to start your cable row. Start by purling the first six stitches. Be sure to give a slight tug after each one. This will help take some of the strain off of the cable stitches, and prevent any holes, tension issues, etc. Try to get a cable needle in the same size as your knitting needle, otherwise, your gauge might not turn out right. If you don't have a cable needle, you can use a double-pointed knitting needle, or even a pencil or pen. Keep in mind that this may cause some gauging issues. Where you place the cable needle will determine which way the cable will twist:  If you want your cable to twist to the left, leave the cable needle in front of your work. This is known as the front cable. If you want your cable twist to the right, place cable needle behind your knitting kneels; this is known as the back cable. Leave those first three stitches on the cable needle. You will knit them in the next step; this will create the "twist." Avoid pulling too tightly. Instead, try to keep your cable needle as close to your project as possible. This will make it easier to knit later on. Put the cable needle off to the side, and purl the final stitches off of your knitting needle. Once you have finished, flip your work, so that the needle with all the stitches is in your left hand, and the empty needle is in your right hand.

Summary:
Purl the first six stitches. Slip the next three knit stitches off the knitting needle and onto a cable needle. Move the cable needle to the front or back of your work. Knit the next three stitches off of your knitting needle. Knit the three stitches off of the cable needle. Purl the last six stitches, and flip your work.