To get a beautiful flower design or an arrangement, you don't need to show all of these three. The most important here is that your design has ornamental flowers; you will still get a terrific and an artistic flower arrangement.
Next, fill your "real" vase about two_thirds of the way with warm water. Make sure it doesn't leak. Add flower food if available. (Follow the instructions on the packet.) Place a few of your larger blossom flowers in the vase and step back. This will give you a better idea of how to proportion your bouquet. If the stems are too long, simply cut them shorter. If they are too short, try a different container, or add pebbles or marbles to the bottom of the vase to elevate your flowers.