These cute mushroom instructions can be found in several places on the Internet. This is not my own idea, but wanted to share here, in case you have not seen this idea yet.
I made ours by cutting wine corks in half, using a mini hacksaw (on a protected work surface).
I used a small nail, and hammered a hole in the center top of each bottle cap (over a scrap piece of wood). You can use a drill too. I then painted metal bottle caps red, using acrylic paints. When that paint dried, I used white to make the dots. After the white dried, I sprayed the caps with a gloss seal.
To assemble them, I took a simple metal Christmas ornament hanger and straightened one end, leaving the other end as a hook.
I used a drill and bit (size 7/64), and drilled half way into the center of the cut wine cork (cut side). I inserted the straight end of the hanger into the top of the bottle cap. I applied strong glue to the cut end of the cork I drilled into, and gently inserted the straight end of the metal hanger into it. I gently pressed it into place.
Update: We found that a simple kitchen knife, such as a small cheese knife, works great for cutting wine corks.