The method VC1111 describes is very simple to set up and use. I read an article a few months ago (Field & Stream, I believe) about setting up this very same safety system. The only difference is that this article used the Prusik knot to attach your harness to the rope (for you Summit users, the Prusik is the looped knot that you attach the harness to on their SOP). But I agree with VC1111 that the ascender would be a better way to go. The Prusik works well if you keep an eye on it and make sure that it stays tight and it works best when the rope is dry. Ever notice how that knot slides down your rope & won't stay put when you're hunting in the rain?

Here's another link for those looking to purchase rope, carabiners, ascenders, etc. WT Sherrill Arborist Supply They're an arborist supply company so they carry all sorts of tree climbing gear, tree cutting & trimming gear, etc.