Cutting off the top few feet of your Norfolk Pine will destroy the beautiful symmetry of the tree. My best suggestion is to cut off the very tip of the new growth, where the star cluster of foliage is, and four adjoining inches of trunk. Plant the cutting in a small pot, moisten well, and place it in a plastic bag for a few weeks while it develops roots. Your existing Norfolk Island Pine will never look quite as good again, but you will have a new plant to eventually replace it.