I will show you how to attach to a headband after you are done(even though this is an ugly band and not what I actually attached this piece too).
You will cut a slightly smaller piece of felt.
Cover the smaller piece of felt with glue(be generous, but not TOO generous).
Cover the band, along with, where the felt with flowers will be touching it as well.
Squeeze together tightly and set aside to dry.
I would catch it in between a drawer(or standing up between books so it's not laying on it's side(this will cause the felt to slip).
A small note about the glue I use.
I use loctite crafters glue(indoor safe version).
It has the best hold I have found without the smell of the other adhesives.
Honestly, I don't recommend using the alternative smelly versions unless you plan on making these outside(and letting them dry outside too).