The seeds are contained in the flower, which will turn into a seedhead or pod at the end of the season.

Usually seed is only saved from non-hybrid plants. If the plant is a hybrid, the seeds will probably not run "true" to the parent plant. But if you don't mind taking a chance, it IS free seed.

Pick the plants you want to save from based on characteristics you prefer, such as color, size, disease resistance, etc. For example, if you want early blooming, watch your garden and mark the plants that have this quality.

Make sure the seeds are mature and dry before saving. Tap on the stalk. If they rattle they are ready for drying. If it is a plant that has tiny seeds inside a pod, open a pod from a stalk that is turning brown. If the seeds are brown or black they are ready to harvest.

When the seed capsule starts to open seeds are nearly mature.

To collect seeds which are "spit out" when the pods are mature (like poppies), tie a paper bag over the top of the seedhead.

You can also cut the seedheads when the seeds start to turn brown and hang them upside down to mature and dry.

Harvest seeds late in the day after a few dry days, to ensure that they are good and dry starting out.

You may be able to just shake seeds into a bag. It is better to do this instead of pulling them off since you will be getting mature seeds this way.

Store seeds in a glass jar, baggie or paper envelope. First you must dry them thoroughly for several days (I dry mine for a whole week in an air conditioned room). Make sure you label each container with as much detail you can provide.

Keep your seeds in a cool, dark, dry place. The refrigerator is a great spot if you have room.