These are incredibly easy and can be done in under 5 minutes. They look like you put waaay more effort into them than you actually did. You can use almost anything for the cord.
First, flatten out the bottle cap a little with needle nose pliers
Second, pick out an image and trim it so it fits into the bottle cap. it helps if you trace beforehand
Next, cover the image in packing tape to "laminate" it
Then, fill the bottle cap with hot glue
Press the image into the bottle cap and smooth it out, make sure there aren't any bubbles or gaps
Viola! add a hook of some kind on the back and then string it on any kind of cord, ribbon, lace, etc that you want! you can also glue it onto a ring or pin