Summer of 2015, I collected seeds from a parsley plant in my garden, enough for about 5 years of parsley.
At the end of 2015's summer garden, I left one carrot in the ground in my garden's carrot patch, to go to seed this summer. Carrots are biennials, meaning it takes 2 seasons for flowers to develop.
Over winter, the green top of this one carrot died back. In spring, it put on new growth. And I left it there, to flower. All summer long, this carrot's flowers have been developing seeds for me.
This past weekend, I clipped the most dried seed head. (When the seed head turns tan, and the seeds look dried, that's when it's time to gently cut the seed head from the plant.)
If you look at it up close, you can see there are hundreds of seeds on one seed head. And that one carrot produced 7 seed heads. Carrot seeds remain vigorous in their germination for about 3 years. Some carrot seeds will still germinate beyond that 3 year mark, but the percentage of viable seeds will decline.
I shook the seed head over an open paper bag. The remaining seeds will continue to dry, with the seed head, upside down and inside the paper bag, just sitting on my desk.
In a couple of weeks, I'll rub the remaining seeds off the seed head, and store this paper bag, inside my seed container, for planting next spring.
(If you live where weather is severe enough for carrots to rot in the ground over winter, here's an old-timer's tip for collecting carrot seeds. Dig up 5 or 6 of your best carrots, being very careful not to bruise the skins. Clip the green tops to 1-inch of the top of the carrot. Store in moistened sand in a cellar, until springtime planting. Replant the carrots, and allow to flower. Collect seeds, as per above.)
I choose a different vegetable or two each year to collect seeds from. It's easy. It's thrifty. And it fits my idea of fun.