Snip off a half-inch from the stems of the flowers and put them in a bucket filled with ice water. The flowers will re-hydrate, which means their stems will be stiffer and easier to handle and they will last longer in the bouquet. Keep the flowers in the bucket for about an hour before making the bouquet.
Cut the flower stems all to the same length. A good length is 12 inches. It's easy to work with, carry and long enough to put in a vase of water. Replace the flowers in the bucket after you've cut the stems.
Take one rose and hold it in your non-dominant hand. Surround the rose with a circle of three carnations. Hold the flowers firmly, but don't crush them. Surround the three carnations with five to seven dahlias. Exactly how many depends on how big the dahlias are. Surround the dahlias with alternating roses and carnations. You should have a rose in the center ringed by carnations, which are ringed by dahlias and a final ring of carnations and roses.
Fill in gaps by slipping in stems of gypsophila. Loosen your grip on the bouquet just a bit to allow the stems to slide in.
Adjust any flowers that are higher or lower by pulling down on their stems. Cut the stem so they are all the same length.
Continue the bouquet when it gets too big to hold in one hand by putting a rubber band around the stems. Place the stems in a tall container like a Mason jar and add additional rings of flowers. The edge of the Mason jar will keep the flowers in place.
Slip a rubber band around all the stems about 4 inches from the flower heads. Wrap ribbon over the rubber band.
Place the finished bouquet in a vase of water.