What you’ll need:
50 to 100 feet of 1″ tubular webbing for a 25-50 foot slack line
2 48″ or longer slings for the anchors.
2 oval carabiners
2 locking carabiners
Then there's a muenter hitch a couple of clove hitches and a 4:1 mechanical advantage and pull, maybe the video explains it better.
I have also seen it done with a ratcheting cargo strap, but for climbers that just seems wrong.