If you want to harvest seeds, best thing to do is keep the plant well watered and healthy even if it looks unpleasant wait for seed pods to grow from spent flowers. Then when they get bulbous and dried out looking ( which may take some time) Then the seeds are mature. Although I am no expert on this so maybe someone else has a suggestion.
Heather

I've been keeping the seed heads from my pansies forever but never bothering to use them. Tonight, while collecting seed heads, I noticed that one of the flower heads had opened into three sections revealing many pretty little brown seeds. One other also had brown seeds. Of the others, some had white seeds and some had not yet developed seeds. I suspect that only the brown or tan seeds are viable, that the others are unripe. Just an opinion. It'd be fun to plant them and see.