Middle Flower Stem
You may have to place another flower or two before you have enough of a "grid" to place the main focal flower (the biggest, such as a opened lily) in the center. If you have two large flowers (two lilies, for example) you may want to place one top and slightly off to the right, then place the left (a little shorter) off and to the left.
The center flowers should be a little taller than the side flowers, in order to create the rounded look from the side.
If the flowers are all the same, just place the flower dead center and slightly taller than the side flowers.
This is the view from the top. If you need an extra flower or to in order to create a grid that will hold the center flower straight up and down, put them in place before adding the middle flower.