Remove any wilted, torn or yellowing blossoms and leaves from the fresh flower stems. If using silk flowers, test them in water first for bleeding dyes.
Cut the stems of the centerpiece flowers on a slant with a sharp knife. The tallest flowers should be an inch or two shorter than the height of the vase. Test the flowers in the empty vase to be sure they are at a proper height.
Arrange the flower stem or stems in the glass vase. Secure a piece of waterproof florist clay into the center of the bottom of the vase. Set the stems of the flowers into the clay to secure them in the container. You can also use florist glue to secure the floral stems to the glass. Apply the glue to the bottom of the cut stem. Hold the stem against the glass to allow it to adhere in place. A third alternative is to use fishing line and small silver weights. Wrap the line around the stem and attach a small fishing weight to hold the flowers in place under water.
Set the vase in place on the table. If using fishing weights, try to position a blossom over the metal piece to hide it. The weight will be on the container bottom.
Gently add small stones or clear glass marbles to the bottom of the container if there is exposed floral clay or fishing weights to mar the look of the submerged flower centerpiece.
Slowly pour distilled water into the vases.