Secure a piece of thick (3 to 4 inches) dry floral foam or craft foam into a decorative container with adhesive. Glue the bottom of the piece securely to the interior of the container. Cut strips of waterproof floral adhesive tape and secure them across the top of the foam and to either side of the top of the container for extra security.
Insert several stems of silk greenery into the foam to cover the top of it. Leather leaf, ivy and short greens will be good filler material to disguise the foam.
Hold the stem of one of the tallest flowers you have chosen for the floral arrangement against the container. It should be two or three times taller than the container height. Roses, gladiolus, hydrangea, mums, snapdragons and delphinium may be appropriate heights for a tall arrangement. Cut a piece of thick wire with wire cutters. Securely wrap the wire to the bottom of the silk flower stem with floral tape if you need to add to the height of the piece. Insert the first flower into the center of the foam. A silk stem should penetrate at least 2 inches down into the foam to secure it in place.
Insert an even number of long stem blossoms into the foam around the first flower to create the shape of the arrangement. Some of the outer flowers should be set in at an outward angle to give greater width to the arrangement.
Insert at least two to three dozen silk flower stems of varying heights into the foam to fill in the floral arrangement. Turn the container to allow a view from all sides of the arrangement. Fill in any gaps with another flower or stem of greenery.
Take the arrangements that you prepare to the gazebo and set them in place. Walk around the gazebo to look at the effect that the flowers create. If any of the arrangements need more flowers or greater height, add the materials needed to complete each piece.