By "Xenification", I understand to boot Slack from a Xen kernel (as a host, or as "Dom0" - in Xen terminology). Then you are also looking for a Slackware image (to boot as a Guest, or "DomU" - in Xen terminology).
Mario's webpage has it all. Check it out:
http://slackbuilds.slackverse.org/_new/xen/
Read the documentation and if you have specific questions, post them here.
You will need a Slackware DVD. I would prefer the 64 bit for Xen, but it should work for both.
Chris