Blocking in florist foam for your altar flowers is the first step. Once the brick is completely saturated in water treated with a cut flower food, you'll need to place it in the florist urn and cut it to the proper shape.
One brick of foam is sufficient for a large floral design that fits nicely as an altar spray. Super large designs may require blocks of Oasis rather than bricks.
Push the brick firmly down into the urn. For different sizes of containers you can trim with a knife, but these urns are a perfect size for one brick and need no trimming.
The foam will extend high above the rim of the container as shown.
Inexperienced designers cut the foam level with the rim of the container, but this means that all your stems have to stick upward and outward.
It's better to cut a couple inches ABOVE the container rim. This allows your stems to be inserted horizontally and at a downward slant, giving a more pleasing look to your completed design.