If you use a water loving flower like hydrangeas, you will have to prepare these shortly before going to your venue to decorate the cake. (This cake I actually decorated at 2:00 while the wedding was taking place).
It would be wiser to choose flowers that are hardier and hold up well even when not in water. There are water spikes for for water loving flowers, but I don't like to use water tubes unless I have to. They insert big holes in the cake and (horror!!) and I would hate to drip water droplets on a wedding cake.
Remove the leaves (you will actually create toothpicks with leaves as well - so don't discard).
Pinch off any browned or wilted florets.
Hydrangeas sometimes develop brown spots during the dry pack process. This is from the flower petals being pressed so tightly together and bruising begins.