This is easy peasy. If you have really small petals you can place them on top of little cakes or financiers. Mine were too big for that so I scattered them around the pile of mini lemon drizzles (yet to post). They also look lovely scattered over a silky buttercream iced sponge.
I ordered my roses from http://www.only-roses.com where they assured me on the phone these premium Ecuadorian roses are organic (thats what you want as they aren't sprayed with yukky stuff and therefore wont taste yucky) but I still advise giving them a rinse.
So after you've rinsed.. brush with a pastry brush a thin layer of egg white whisked with a little water on both sides. (I used pasteurised egg white)
Now just sprinkle with fine caster sugar until covered.
Lay on a piece of parchment paper and let dry for a good 10 hours. I actually put my oven on 100C whilst brushing, turned it off and then slipped in the tray of petals so you've got a guaranteed dry environment.
When dry keep in an air tight box and I reckon they'll keep for ages. Make now for Valentines day!?