Purchase your flowers just before you plan to make the arrangement. In this way, you will be assured that they are as fresh as possible. If you live in a large city, there may be a floral market in your location. If not, you can purchase loose flowers from florists, grocery stores and big box stores.
Bring a plastic bag, bottle of water and rubber band with you when you purchase flowers. Some locations offer a water-filled carrying bag, while others don't. Place the flower stems in your plastic bag, fill it with enough water to cover the stem bottoms, plus an inch. Wrap a rubber band around the bag, several inches above the water line.
Fill your sink with warm water, as soon as you get the fresh cut flowers home. Place the stem bottoms in the water. Cut, at an angle, about an inch off the bottom of the flower stems, while still in the water.
Pour a packet of floral preservative (from florist, floral supply, grocery store or big box store) into a clear container. It can be a vase, but also can be a container from the kitchen cupboard. Floral preservative will quite often come free with your flower purchase.
Fill the container with polished stones, marbles or florist marbles, one-half to one-third to the top. Add water to fill up to one-quarter of the way to the top. The stones and water will hold your flowers in place.
Place your flowers, cutting them underwater, as you decide the height needed. If it is a low container, start with greenery around the bottom, followed by the focal flower. If it is a tall, thin container, start with the focal flower, in the center. This will be the largest, prettiest flower you have.
Place the rest of the flowers and greenery, cutting the stems to flow in a cascading manner from the focal flower.