Firstly you will need 2 of the same plants but different clones to produce seeds. When the flowers are open using a brush or tweezers remove the pollen from one flower and place it into the other flower of a different plant and repeat the process back that is it. Soon if successful a fruit will grow and seeds should follow. Depending on the species seeds can take from 2months to six to develop they will usually change color when the are ripe.
I find that warmer weather helps the pollen become lose and easier to collect. Most seeds when ripe can be sown straight away or up to 2 years later after that they will start to lose viability.