You must first soak your Oasis florist foam until it is complete saturated with water, then cut the foam down to the correct shape to fill your pew clip.
To extend the life of your flowers, always use a quality flower food. I use Floralife, following the directions on the back of the bottle. More than recommended will not hurt the flowers, but is a waste of product.
Less than recommended will not give your flowers enough nourishment. Remember your flowers are still a living thing, needed essential water and nutrients to stay alive (especially after being in a dormant stasis for so long).
I also use Quick Dip (a quick re-hydrator) to "wake up" the flowers. Learn how to process your flowers correctly to assure the longest life possible for your flowers.
Once the foam has been "float soaked" and has sunk to the bottom of your water container, press the pew cage lightly against the brick for the outline of the cage size.
Thus you will have an accurate guide for cutting the needed foam.