Soak the floral foam according to package recommendations, if you will be using it to secure the flower stems in the vase. Insert the floral foam into the vase after the soaking period.
Create the height of the arrangement with the stock flowers. Trim off the end of one stock flower so the height of the stem equals about three to four times the vase height. Insert the stock stem in the center of the arrangement to be the highpoint of the arrangement.
Define the width of the arrangement with two more stock stems. Trim the bottoms of these stems to make them roughly the width you desire for the arrangement -- make the width narrower than the height. Insert one stock stem facing the left and the other stock stem facing to the right to create the width.
Add the focal flowers to the arrangement, adding an odd number of focal flowers spread over the central area of the arrangement. Place these flowers so they distribute the color and focal points in the middle and just below the center point of the arrangement.
Fill gaps in around the edges and in between the focal flowers with filler flowers and foliage. Use small groupings of baby's breath, Dianthus and ferns to add complementary texture and color to the arrangement.
Step back and assess the arrangement. Make any necessary changes to the placement of the flowers.