Cast on normally and then you use the two ends to knit. Remember that when you knit flat your working yarn will come from the stitches on your left needle. When you get done just turn it around so the stitches are on the left with the working yarn coming from the end again.
If you have a lot of stitches on the cable you can accidentally join and knit in the round if the yarn is in the wrong position to knit. No big deal, we've all done that at least once when we were learning.