Step 1: What you need
1 in Nylon Strap (the kind used on backpacks)
Drawstring Stoppers (like what you might have at the end of a hoodie string)
A Carabiner (AKA Spring Link)
A lighter will help you melt the ends of your cuts
and Scissors and a ruler (not pictured)
I actually salvaged these parts to make this, cord I had laying around, the strap was cut off an old messenger bag and the drawstring stoppers were from an old coat. Never throw these plastic pieces out. Even if you buy all of them it would still only be $5 or $6.
Step 2: Bottle Carrier Loop
First, cut a length of cord. Mine is about 15 inches because I have a large mouth water bottle. If you are only carrying a store bought bottle or a sport drink you can use about 10 inches.
Second, Tie the ends to make a loop. (go ahead, use a square knot)
Finally, push the loop through the drawstring stopper. You may need to use a toothpick or something like that to push the cord through.