Although growing dahlias from seeds is certainly more difficult and takes more work than simply sticking a bulb in the ground, it can also be a lot more fun and rewarding. For those that wish to make propagating into a hobby, harvesting seeds and sowing them is the way to go. It is more of a challenge getting those small seeds to grow and the result might be so great that it will surprise even you. And remember, there is nothing compared to telling your neighbours that the garden is all your doing, from scratch (so to speak!)
First of all, it is best to find out which of the seeds that are usable and the easiest way to do that is to pregrow them a little. Place a folded damp piece of paper towel on a plate and then spread out the seeds over it, then take another piece of folded damp piece of paper towel and cover the seeds with it. By placing this whole contraption in a dark and cool place the seeds will think that they are in the ground and start to grow. In just a couple of days it will be obvious which seeds to keep and which have failed to germinate and these can be thrown away as bird food. Just remember, the paper towels need to be kept damp but should never be allowed to get soaking wet as the seeds might start to rot instead of grow.
The seeds that passed this first part will then be presowed in small containers filled with fine sowing soil. This can be bought at any local garden shop. Each container should have two dahlia seeds in them, as far from each other as possible, just at the wall, so it will be easy to tell which is which. The soil also has the best drainage next to the walls of the container and that is important for growing dahlias. The soil should not be watered from the top but instead get its water through holes in the bottom by standing on a plate of water. Make sure that the temperature is right and that the containers get enough but not too much sun. If the surface seems too dry a simple spray bottle will be sufficient to give it some moisture. Once the dahlia seeds have started to grow it is possible to choose those that seems to be the strongest of the plants - it will be Darwinism, survival of the fittest, in your own home. Once that is done it is simply a question of planting your dahlias where they belong, either in a larger pot or outdoors in your garden.
The exciting thing about growing dahlias from seeds is that you never know what to expect. Because the dahlia has so many different genes controlling its appearance (eight instead of the normal two) the dahlia that gets planted can be any kind of dahlia and will most probably not look anything like its parents. Even using a q-tip (during the actual growing season) to perform the pollination on your parent flowers is a safe way to try a little gene experimentation because propagation always has a great random element to it. So experiment and have fun, you can make your garden entirely unique by creating your own flowers through seeds.