You can see a definite crossing of the stems as you add flowers to the bouquet. It will be this crossing that fans the roses outward and creates your rounded shape.
Greenery should always compliment the flowers - not overpower them. Don't go overboard with adding greenery.
Keep alternating as you go, changing between myrtle sprigs and tree fern stems.
You'll know you are doing it correctly as you see your lower stems fanning out in a definite "V" pattern.
Be sure to keep this pattern shape when taping the stems with the Bind It tape.