Well you can't really do this. Most geraniums are grown from cuttings and their flowers do not produce flowers. Now, there are seed-produced geraniums, but I don't know if you have those varieties. They might produce seed, but they would probably not germinate or would not look like the parents because they are all hybrids. You can buy geranium seed in the spring from seed companies for next to nothing and start them early indoors under grow lights, but harvesting your own seeds and growing plants from them is very difficult if not impossible. The best way to preserve the plants you have now is to take cuttings and root them, pot them, and place in a sunny window till spring.