Get felt. Cut out a strip that is 2 by 3 inches, and another strip that is about 3/4 inch by 2 inches.
The skinny piece in your photo is not an accurate representation of the measurements given above.
Sew a double running stitch in the middle of your bigger felt piece.
Pull the string to ruche your felt piece so that it starts to look like a bow. Be careful not to pull too hard - you don't want your string to break.
Wrap your smaller piece of felt around the middle of your bow, and sew it together in the back.
You might also want to sew it down to your bow a bit so that it has more stability and won't run around.
Don't worry if your sewing is ugly back here. No one's going to see it.
Insert a hair pin through the opening between the bow and cincher.
If you want, you can glue yours down with hot glue [which I don't have, sadly] or you can even use one of those snappy hair pins. Remember those? From when you were a kid?