Best wait till the seeds are ripe in their pods before collecting? Not when they're still green. When you've got viable seed let them dry in a paper bag or envelope & sow in Spring. If you keep an eye on the seed pods you'll see when the pods split open to give up their seed? Then snip them & store til next year

I agree collect the seed when the pods turn a beige/brown colour. keep in a cool dry place and sow in March under glass[window sill if not got a greenhouse] they don't need any heat. or April sow in the open ground where you want them to grow. don't forget the seed will not always produce flowers of the same colour as the parent.
I love stocks and often start Brompton stocks off to use instead of wall flowers.

That’s great thank you. Will keep an eye on them. They are just outside my back door so when I get a minute I have a quick sit down with a cuppa and take in their scent!