Using a Taped Flower Grid
If you find just using the roses alone difficult to design, you can use clear tape to separate your flowers. It is concealed by the roses and greenery later.
Regular clear tape on a dispenser can be used in a pinch, but is generally a little wide for floral work. (Do not use the tape that has a "frosted" look). Pull the tape across the vase and overlap the DRY sides of your clear vase.
You can pinch the tape together to create a narrower grid if necessary.
Just use your thumb and forefinger to pinch the sticky side of the tape together.
Professional florists use clear gridding tape that is much narrower. The vase must be dry on the outside for the tape to stick to it.
Create a grid similar to this one.