Via the lovely Miss Megan,
You'll need obviously a bottle cap, fabric, hot glue, some stuffing, a needle and thread and ribbon or other fabric scrap to cover the cap.
1. Measure out your circle, I use a ribbon spool but you can use a rim of a glass or other circular object. If you actually have an artistic bone in your body draw that circle yourself.
2.Cut out your circle.
3.Sew a running stitch all the way around the circle.
4.Plop a nice size of stuffing down in the center.
5.Tug on the thread to gather your stitches around the stuffing.
6.Fill your bottle cap with some glue. Check out that snazzy action shot.
7.Stick that sucker down in there.
8.Cover the cap with some ribbon, scrap fabric, paint it, or leave it be.
And voila! Sit back and enjoy your work.