Get a tulle, a ribbon and a hair band. O preferred them to be black.
Fold the ribbon and place it aside.
Grab your tulle...
...and just tie it on your hair band with a weak knot.
Now take the black ribbon and push it trough the knot.
It should look like this.
Now just firm you tulle, so it would hold the black ribbon well...
And you are done. The best thing is that you can remove the bow if you get bored of it and make something else;)