Hi,
Rosemary seed has a very low germination rate so most people opt to root cuttings.
Thyme seed will be very tiny dark specks that will come out of old flowers. Wait until the flowers are dry and then try crumbling the remnants of a flower cluster onto a piece of paper. If you get little specks rolling around, you know you can pick all the flowers and harvest the seed.
Remy

Hello

I can't give you any tip on seed saving for thyme and rosemary.

I have however grown 3 rosemary plants from seed. It is true that the rate of germination is low, so you need to sow many seeds to make sure that you will end with a plant or some plants. So that is useful to know, I think.

Another useful thing to know is that it can take a little while to germinate - at least for some seeds. I sowed the seeds in winter and one of the seeds germinated relatively qucikly. Much later, in summer, I got another 2 seeds to germinate. I had in fact forgotten they were there and I did not expect any more rosemary, but i had not thrown the sown seeds away. It happened in the summer when I had put a lot of my pots and planters outside in the balcony, so probably the warm weather helped. So that is another thing that might be useful to know.

Good luck!

Sálvora