Of all the flowers in your garden, the dahlia is perhaps the easiest for the homeowner to hybridize—and the most difficult. This is because dahlias are what is known as octaploid, rather than diploid, when it comes to chromosomes. They have four extra sets of chromosomes, so the mix of chromosomes you end up with is much more difficult to predict. However, the good news is that if you breed two dahlias with similar properties, then you have a good chance that the same property will show up in their offspring.
Find two healthy dahlia plants in full bloom with the trait that you want to breed for. Both plants should have the same trait you are looking for, and you should breed for only one trait at a time.
Wait for one of the blooms to open up its center and begin to show its pollen-producing stamen.This should occur before the second flower has opened up its center completely. Cut off the bloom of the first flower (the male) with the open center and remove all of the flower's petals. Place the flower in a glass of water and set it in a sunny window to wait for the pollen to form on its stamens.
Cut the toe off a nylon stocking and cover the head of a second flower before it fully opens. This is to keep bees and other pollinators from getting to the stigma (the female parts of the flower) before you can pollinate the flower yourself.
Remove the stocking from the female flower when the bloom opens and the stigma are showing. Remove all of the flower petals from the female flower.
Pick up the male flower, hold it upside down, and gently rub the pollen-covered stamens back and forth across the other flower's stigma. Rub the pollen in both directions until you are certain as much pollen as possible has been transmitted from the male flower to the female flower. Cover the female flower with the stocking once again.
Repeat Step 5 one additional time the following day, and then cover the female flower with the stocking and water it regularly (keeping the soil damp but not soggy) for three to four weeks until the seed pod has formed and turned a light straw-green color.
Remove the seed pod and allow it to dry. When dry, open the pod with your fingers to reveal the seeds. Seeds can be planted in pots filled with soilless germinating mixture in March and kept in a sunny window until strong enough to plant outdoors (once all possibility of frost is gone).