White PVA glue (such as Aileen’s) works well.  It can be used to glue the flowers as well as to coat them, if desired.  Coating flowers with glue after the design is completed on agreeting card, for example, will make them less fragile for mailing and help prevent them from fading.  Disappearing purple glue sticks (such as 3-M) work well for short life projects, such as greeting cards.  You will be able to see where you applied the glue, apply your flower, and any excess glue will dry clear.  You can even do without glue for some projects, such as tags and bookmarks, by laying out your design on your paper and then very carefully laying clear contact paper over your project piece and trimming off the excess.