Its just like a soft carabiner...

Open the loop, slip the big knot out of it, run the loop through whatever you are going to secure, put the loop over the knot again and milk it back tight(close the loop under the big knot) ..

They are simple, light and very strong. I use 4 of them in my hammock/SRL setup, and use them on my tarp prusiks and pretty much anywhere else I need a carabiner.