Step 2: What you need.
empty soda/water bottle
Knife or razor blade
Dremel, for touching up the saw cut
not pictured - a clothes iron is nice, but not necessary
a US 5 cent piece, otherwise known as a nickel
Step 3: Prepping the bottle
Now, use the knife to remove the little plastic strip around the bottle neck.
Step 5: Finish with Dremel
I left a very slight raised "ridge." I find this makes it easier to use.
Step 6: The nickel
Step 7: Gluing
If you mess up.. like I just did... then you can place the whole shebang on top of your overturned clothes iron at medium heat. Don't use the highest setting, like I did... I just found out that the plastic turns white when it gets too hot! :)