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.
1
First, flatten out the bottle cap a little with needle nose pliers
2
Second, pick out an image and trim it so it fits into the bottle cap. it helps if you trace beforehand
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Next, cover the image in packing tape to "laminate" it
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Then, fill the bottle cap with hot glue
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Press the image into the bottle cap and smooth it out, make sure there aren't any bubbles or gaps
6
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