Here is a video on how to attach the finger loop.
To attach the finger loop turn the bottle over and make sure the lid is closed.
Pinch the string portion of the loop and feed it through from top to bottom on the bottom most metal pin on the back of the lid.
Pull the string through and feed the rubber portion through the loop that you just passed through and then pull down tight.
Now you have a convenient way to carry your bottle with just one finger.