Use a little tylose mixed with water, or if you can't get tylose, use meringue powder mixed with water.
You can also just thin down a bit of fondant with water to use as a glue. Works great.
I always use tylose glue! I used royal icing before I knew about tylose glue, but it had a tendency to leave a bit of a residue around the edges... the tylose is nice because it will allow your piece to lay lat on the fondant without any goopy glue sticky out!!
Using plain water with a paint brush works great too.
plain water or gumpaste glue-
I brush a little water on the back of the flower. Be careful not to put too much because you don't want it to drip.