Fill the vase about three-quarters of the way with water. Place a rose in each corner, crisscrossing the stems; the flower heads should protrude about an inch past the edge of the vase. Fill in with more flowers along the edges, weaving a web of stems. For successive layers, cut stems slightly longer and slip them into the webbing. The last roses you add, in the center, should be about a half-inch taller than those on the edges.