One of the first things to do is collect seeds.
You can only do that when the seeds are ripe.
So read about your plants and find out how you can see the seeds are ready for harvest.
It's best when you harvest the seeds after a few dry days, but that's not always possible.
Put them on a flat surface and allow them to dry.
First take any leaves and twigs away, and if possible everything else around the seeds too.
Sometimes seeds are in pods that are very easy to remove when they are dry, then dry the seeds and the pods together and remove the pods later.
Let the seeds dry more time before you store them.
Maybe in an envelope with the name on it.