You will have to switch to double-pointed needles at this point; or work with two circulars.
There are videos in the Advanced Techniques section for both methods.
It's really easiest to learn to use dpns under these circumstances, where you already have stitches on the needles. You can either knit onto the dpns (about a third of your stitches on each) or just slip the stitches on the dpns.
You might want to arrange them so your decreases are at the beginning of each dpn so make it easy to keep track.