Turn your vase upside down, holding it in mid-air, so that the opening is pointing downward, and press the rim (opening) all the way down into your piece of floral or regular foam. This will result in a perfect cutout of the foam that fits your vase. Use any piece of floral foam or regular foam at least a few inches thick and with a large enough surface area to fit the rim (opening) of your vase.
Remove the perfectly fitting foam from your vase only if you intend to us real leaves and flowers. This will allow you to first put in water. Then reinsert the piece of foam. If using fake flowers, leave the foam piece inside.
Penetrate the stems of the flowers, leaves and vines through the foam, one by one, to make your arrangement. Arranging flowers is largely discretionary and most designers use a trial and error method, inserting and removing flowers or rearranging them until the arrangement is pleasing to the eye.
Adjust the height of the flowers by cutting the stems accordingly and bundling several small flowers together where possible.