Fill the vase three-quarters full of warm water. According to the Alabama Cooperative Extension System, using warm water helps the flower absorb flower food and water more quickly.
Dissolve a packet of flower food in the water. If your roses didn't come with a packet of pre-measured flower food, substitute one tablespoon each of household bleach and white sugar.
Fill a bowl with warm water. Place an empty vase on a table. Measure the height of the vase.
Lay the roses on the table in a row. Hold the stems below the water in the bowl and cut two to four roses (depending on the amount of roses to be arranged in the vase) three inches taller than the vase height. Cut from the bottom of the stems. Remove any of the leaves that will be below the waterline in the vase. Pinch them off with your fingers.
Measure and cut the remaining roses two inches taller than the vase height. Remove leaves that fall below the vase's waterline.
Insert the taller roses into the center of the vase. Hold the roses in position with your hand. Place the remaining roses around the center roses in a row along the inside of the vase. Release the center roses. The center roses will be slightly taller than the outer ring of roses, creating a pleasing cascade from the center down to the vase edges.