When I learned Ikebana as an adult, I was really fortunate to have had an old_fashioned sensei (instructor) who not only taught me the theory and techniques, but also immersed me in the respectable manners and ways of the good old days. It is through my sensei that I learned that Ikebana goes beyond the typical art form, and that it is more of a way of life; in fact, Ikebana is also known as kado, which literally means "the way of the flower."
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.