For every bottle cap, you'll make two of these circle shapes. There will be one minor difference in the circles: you only need to leave an extra-long length of yarn on one of them. On the other one, the yarn tail at the end can be shorter, measuring around 6 inches so that you can weave it in comfortably.
This photo shows my bottle cap and my two circle shapes. As you can see from the photo, I have a tapestry needle threaded with the long length of yarn that's attached to one of my circles.
For those who don't know, a tapestry needle is a needle with a large eye; this particular tapestry needle has a large enough eye that I can thread yarn through it.