Creating a bubble bowl grid is the easiest way to design if you are just beginning. This helps keep the beginning flowers in place. Use clear floral tape, which is available in either 1/4" or 1/2" widths.
I'm using white waterproof tape for the demonstration, since it shows up much better in the photos. You will want to make the grid out of clear tape, however, to conceal the mechanics. Be sure to add the water and marbles before gridding the vase, or splashed water could loosen your clear tape.
I have about a 1/3 bunch of bear grass wired down the stem.
You will tuck the bear grass in later - but for now set it aside until we get the other flowers in the vase.