Once you have gone to the trouble of covering a cake with a smooth, even layer of fondant and spent plenty of time creating 3-D flowers also molded from fondant, you don't want to destroy all your hard work by attaching the flowers incorrectly. Not only could you wreck the surface of your cake, you could also break your flowers or have the forms droop and fall later on. Knowing how to properly attach the flowers will ensure that your cake stays together in the hours or days after you have completed it instead of coming apart.
Allow your flowers to dry after creating them, at least overnight. Fondant does not spoil, so feel free to make them as far in advance as you want.
Mix 1/4 tsp. water with 1 oz. fondant. It will be a soft and sticky-feeling paste with a somewhat thinner consistency than the regular fondant. It the mixture isn't very sticky, add a few more drops of water. Put the thinned fondant in a pastry bag.
Using the pastry bag, pipe onto the fondant-covered cake the thinned fondant where you want to attach your fondant flower.
Gently press and hold the flower against the cake until it sticks. This should only take a few seconds.
If the fondant gets too hard as you are working and you have trouble piping it out out of the pastry bag, microwave it at half power for a few seconds to soften it.
Buttercream can also be used to attach the flowers, although it can be a little messier.
If you want a spray of flowers coming out from the cake, consider attaching them with wire. Insert the wire into the fondant flower when you make it, and allow the fondant to dry around the wire. When the time comes to attach the flower to the cake, simply insert the wire where desired.