First and foremost, this is an ad-hoc, not-really-reusable device. It is intended to fill a last minute need and will not win any awards for its aesthetic qualities. That said, sometimes you just need a cocktail shaker on the fly, and in those situations, this is the device for you!
Step 1: Gather Your Materials
You will need:
1 plastic bottle
Step 2: Cut Off the Top of the Bottle
Be safe about it! You'll need more than a stiff drink if you chop off a finger.
Step 3: Keep the Top!
The top will be used as a measuring device when you make your cocktails, assuming that you do not have a shot glass on hand.
This will, of course, require that you alter your recipes from being measure in ounces to being measured in parts. If, for example, you are making a negroni (which, traditionally is 1.5 oz sweet vermouth, 1.5 oz. Campari, and 1.5 oz. gin), you'll need to think of it as one part (i.e. one bottle cap) sweet vermouth, one part Campari, and one part gin.
Step 4: Create Visual Estimates
In order to close the bottom part of the bottle, you will need to make two slices down the side. In order to estimate how far those slices should go, I'd suggest folding one side down and estimating how far apart and how long the cuts need to be.
Step 5: Slice & Dice!
Create two cuts, according to your earlier visual estimates.
For reference, I place my cuts about 1/3rd the circumference of the bottle away from each other, and 1/3rd of the length of the part of the bottle
Step 6: Make Sure You Sliced & Diced Well
Before putting any ice or alcohol into your shaker, I suggest folding the pieces over as pictured above. There should be a slight gap between the section that folds over and the bottle. You will cover this gap with your thumb when you use the shaker, so be sure that it is an acceptable size!
Step 7: Add Ice & Liquor
Just as a reminder, the cap of the bottle can be used as a measuring device if you do not have a shot glass or measuring cup on hand.
Step 8: Tape & Shake!
After taping down the sides like you are wrapper a present, you are just about set to go! Now, just hold your thumb over the gap between the top-flap and the bottle and shake up your cocktail!
Step 9: Serve!
Now, take your thumb off the gap, and pour into a glass!
The shaker can be re-used by carefully removing the tape from the sides. However, for long-term use, it is best to create or buy a shaker that doesn't require frequent tape application.