Cut it at the start of the stalk. You can frag it one of two ways. First, you can place it a piece of rock rubble and cover it with some mesh (like the bag onions come in), and secure the mesh with rubber bands. The problem I've found with this method is that the Xenia ultimately becomes attached to the mesh, and you end up ripping it off when you pull off the mesh. My preferred method is to place the stalk on the rock rubble, and then cover the base with a good amount of reef gel. I wait 30 seconds or so, then dip it in and out of tank water (I keep a container of tank water in front of me). When the reef glue hits the water, it will harden. Do this until the reef glue isn't tacky anymore, and then position it in your tank. I've propogated tons of Xenia frags this way, and then sell them to my LFS.