Gathering the seeds from your favourite flowers in your garden is a great way to preserve some of your favourite plants, save money on your gardening budget and trade with other gardeners for seeds that you don't have but would like to try.
Deadheading will help encourage your plants to produce flowers longer but it also prevents seed production. If you want seeds you have to resist the urge to pluck those old flowers off, I deadhead some plants and let others go to seed. To know when to gather the seeds is simple enough, if the flower has dried and turned brown it's probably ready to gather, if seed pods have turned brown and you can hear the seeds rattling inside or the pod has started to split it's ready to be picked. If you notice birds and animals eating the fruit from plants you would like to gather seeds from this means that they are ready to go. If you gather seeds too early it takes more time to clean the seeds, if you wait too long the seeds will have already done what they are supposed to do and you will have trouble finding any seeds. For a lot of plants once the petals drop wait thirty days and the seeds should be ready to harvest but it is a good idea to observe individual plants over several years to get an idea of when their seeds are ready.
Choose a dry, sunny day to gather your seeds. Afternoon is good, this gives the morning dew a chance to evaporate. You will need shears, a marker and paper bags, I label my bags before I start (plant name, growing requirements, size and colour) but take extras just in case. Then all you need to do is make yourself comfortable, snip off the dried seed heads and examine them, take only the mature seed pods everything else will be a waste of time and drop them into the labelled bag, if the flower heads still feel damp spread them out to dry so they don't get mouldy. Hang the bags in a cool dry place to allow the seed pods to dry out completely. For fruits you will need to remove the seeds or pits, clean the fruit flesh off the pits by washing, spread out on a newspaper and air dry.
Once the seed pods are dry, gently open them and shake the seeds onto paper towel, a piece of paper or a shallow container, remove debris from the seeds by gently blowing on them or you could use a small fan if you're working outdoors, just don't get the fan too close or it will blow the seeds away as well . Store dried seeds in small, labelled envelopes and seal, then put envelopes in an airtight container such as a Tupperware container, silica beads or rice can be put in the container to absorb any moisture that might occur.
Have you ever collected seeds and when you planted them the next year discover that they don't look like the original plant? In this case you probably have a hybrid and the seeds will not always look like the parent plant. A hybrid is the result of a cross between two different varieties of plant.
Source: http://tomclothier.hort.net/page28.html