Focal flowers are usually the largest flower in the design. Roses are generally very cost effect and come in a multitude of colors. Don't feel like just because this design is all white that those are the only thing you can use to duplicate this design.
Always wash your hands before handling delicate flowers like roses. This and misting your fingers with Finishing Touch help prevent the roses from turning brown prematurely.
Don't be too much of a perfectionist. I like placing some of my roses a little deeper, but most of them should slightly extend out beyond the other flower heads.
This prevents the centerpiece from having too much of a solid "dome" look, while creating a pleasing illusion of depth perception.
The length of your end flowers can be made longer if you wish to make a bigger or longer arrangement.
(Keep in mind that you may need more flowers than listed on the first page of the tutorial.)
If you need especially long centerpieces, the same greening techniques can be used on Oasis Raquettes. These long pieces of florist foam have a built in tray on the bottom and wrapped in a thin polyfilm. It is pierced with tiny holes that all the foam to soak up water (Give them extra time to allow through soaking.)
Then you simply "soak and poke". Very easy to design and great for long table centerpieces. You can place end to end for a continuous design look.
Turn your arrangement around to be sure you are filling in the blank spots evenly.
You should always push the stems gently, working them into the foam rather than poking really hard. Otherwise you may encounter other stems and bruise or mash delicate flower stems.
The design is shaping up nicely, creating a pleasing casual look while retaining the oval shape of a traditional "long and low" centerpiece.