I collect my viola seeds from summer through to autumn. I always have a lot of violas growing in baskets, containers, and borders. You can tell when they're going to seed because the petals curl and fold around the centre before falling off, and you can feel with your fingers as the seed pods get fatter.
Violas need deadheading to encourage new flowers so a lot of the flowers I pick early on haven't got fully developed seed pods, but they do develop very quickly and there's always some you miss.
The pods are often explosive and scatter the seeds widely. For those that you collect, you can just put them in a plastic sealed container, or a container with something on top to stop the seeds from flying out. A lot of the pods will just split open with three sides containing the seeds and you can rub your finger or a fingernail over them to remove the seeds. You can use a wire colander to separate the seeds from the chaff.
Be careful because they are tiny and will go everywhere if you don't do it inside the container. Store them in a paper bag or envelope in a cool dry place, or in a sealed plastic container in the fridge.
If you would like some free advice check out the website for the National Viola and Pansy Society which can be found at floristviolapansy.wordpress
Try this link
Very good website, shows it just right, lots of photo's.
Why collect them just sprinkle them around you can never have too many violas and pansy plants. You can also take little cutting off existing plants, strip the leaves to about 3 pinch out the tips and just stick in the ground after making a little hole with a pencil pop the cutting in and just firm the soil back. Also works with pinks and the like.
Some good tips here. I am saving seed from the best of this years violas.