Bottle cap tacks are very easy and inexpensive to make. Recommended items to use are: 1inch round hole punch, bottle caps, mod podge, glue, tacks, scrap paper or magazines
First choose what you would like inside your bottle cap. I just punched out paper circles for mine.
You could either leave them plain or use stickers to fancy them up. I used number stickers and a bird sticker for mine. Next I put some glue inside the bottle cap to attach my paper to and covered the surface of the paper with mod podge to seal it.
I attached the tacks last so that the bottle caps could lay flat throughout the process.
I have also punched out pictures before to put inside the bottle cap to make more personalized gifts or you could attach a magnet to the back instead of a tack.