It's tough to describe.
Tie off the line to your tree.
Attach the biner thru your belt.
Wrap your para cord 3 times thru the gate of the biner then slide the loops to the solid side. You now have the line ties off to the tree and running to your biner, loops then free end.
As long as you keep tension on the free end the line will grab the biner. When you slack off tension on the free end the line will slip thru the biner. Descend by keeping or loosening tension on the free end f the line. Do not descend steep sloops or anywhere you would not want to fall down.
Try it out somewhere safe first to get the concept and be careful.
Ron Hood had a demonstration in Woodsmaster volume #3.