Taking the tallest flowers in your selection, determine the maximum height you want your arrangement to be and cut the flowers to that height.
Fill the vase with water and add the plant food. Take the large blooms you've chosen and place them near the center of the arrangement, surrounding them with the foliage.
Place the tallest flowers along the back of the arrangement in a general fan shape. Play with how balanced you want to make the fan. You can also experiment with putting some of the taller flowers on each side.
If needed, add more foliage at a shorter height around the blooms to fill the vessel. This will keep the arrangement upright and in place for the duration of its life.
Add the spray flowers sparingly throughout the arrangement, treating them as accents that will catch the eye. Place the arrangement in a window or on a tabletop that needs livening up. Supply the flowers with fresh water.