Continue to add more leather leaf fern from the sides, sliding the stems in between the rose stems.
The more greenery you add, the tighter the grid will form around the flower stems, holding them in place.
I usually use the rule of one piece of greenery (at least) to each flower stem. Tuck some greenery deep in between the inner flowers as well.
A view from above shows how the leather leaf beginning to fill out the design in the empty spots between the roses.
Continue to add leather leaf until you are please with the effect.