Fill the vase 1/3 full of sand, pebbles or, if it is a clear vase, colored marbles. Tall silk flower arrangements tend to be top-heavy. Adding weight in the form of sand or pebbles to the bottom of the vase will stabilize the vase and arrangement above.
Place dry floral foam into the tall vase. The foam needs to almost fill the vase and be a tight fit, not wobbly.
Lay out and separate all of the silk flowers next to the vase. Inspect each stem to make sure all the flowers are attached. Bend and shape each stem so that it appears natural.
Arrange the flower stems while they are laid out on the table roughly how you will want them in the vase. Select one silk flower stem as the focal point of the arrangement, placing other stems on both sides of it. At this point, the color and flower shape is what you will be focusing on.
Hold each stem next to the vase to determine the height it needs to be in relationship to the tallest stem, which will be the focal point. Use wire cutters to cut off stem lengths that are too long.
Insert the tallest stem, the focal flower, into the dry floral foam. Push the stem as far in as you can. Continue adding long silk flower stems around and in front of the focal flower. Keep the overall form of the arrangement in a triangular form, either symmetrical or asymmetrical.
Add filler flowers and greenery in between the main flowers to complete the floral arrangement.