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.
I must give an added note here. These styles of pew cages are getting harder to find than when I was working in a flower shop. This 4" egg shaped cages by Syndicate Sales are a good substitute for the same style of floral design and can easily be attached to the pews with tulle or ribbon lengths.
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, meaning you do not force it under the water. After the brick has sunk to the bottom of your water container to absorb the maximum amount of water, 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.