Hi when a daffodil/narcissus [most flowering plants actually] is pollinated and seed starts to form you have to wait for the seed to ripen. in the case of daffs the ovary is the green swelling behind the trumpet of petals. as the flower fades the ovary will swell. when the seed is ripe it is black and shiny and the ovary will dry and split open. Collect the fresh seed and scatter it where you want the plants to grow. Or scrape back some of the grass and plant on the soil surface and cover with some more soil. It is better sown fresh in my experience but some one else may find otherwise.
if you want to grow it on in pots then chose soil based compost [John Innes No 1 is ideal] and cover the seeds with about 1/2" of compost. place in a cool sheltered spot and make sure they don't get too over dry.

If successful then they may flower for you in the 4th year. But do check local laws regarding collecting seed from wild plants.

see what other members suggest too.

Can't disagree with Sbg's advice. When I collect my own seed I always sow it immediately rather than dry it off. I assume that the original question was about daffodils (?). The seeds take time to germinate and then grow to flowering size. My problem with sowing these straight into the ground is that I am quite likely to weed the seedlings out by mistake before they mature. I grow in pots and then plant out the resultant bulbs once they have flowered.