You should use a cable needle (or you can use a dpn, skewer, chopstick, anything short and straight) that's smaller than your working needle. The mock cables make use of twisted sts over just 2 sts, so may not look the same. If you don't have more than 6 sts in your cable, you can look for cabling without a cable needle. There's a video on the Advanced Techniques page under Decorative Stitches, and a couple tutorials online elsewhere.
sue- knitting heretic