HI, I usually have my flowers made and ready to put on the cake, but if you need to wait a while before you put your flowers on you may need to add a bit of buttercream. I remember once I had chilled a cake and when I started to add the royal flowers they did not want to stick. After a brief panic I put a little buttercream on the back of some and on some of them I took a wet finger and smoothed over the crusted area. Adding buttercream sure can't hurt as long as its not enough to hide some of the flower after you press it in. Good luck! I'm sure your cake will look great and you will do fine!