To knit a little hat for our bottles, you will need:

and one of these:
Just to check you're on the right track, when laid out flat, the hat dimensions should be about 5-7cm along the bottom and at least 3cm high.
With wool being stretchy stuff though, it's difficult to give strict measurements. Therefore, if you can, it's best to try fitting one on a smoothie bottle if you have one to hand.
To help you on your way, we also found a few videos online that you might find useful if you are learning for the first time:
If you'd prefer to print these patterns out to work from, or pass onto friends, we've made each one into handy A4 printable PDF files:

(You can download a free copy of Adobe Acrobat Reader to open the files here if you don't have it already)