In many plants the day length determines flowering and seed formation (not beans and peas).
Nearly 95% of the seeds should be harvested ripe and dry.
Select from the healthiest plants, the plant with the most desirable properties (early or late flowering,..).
Choose not one but several plants or stems where you can harvest seeds.

zaadverzamelenTo spread their seeds as far as possible many plants will make an as long as possible stem with flowers and seeds on top. In the garden we call it bolting. Biennial plants do so only in their second year of life.

Example for lettuce. The crop grows higher up and bolts a long stem that goes flowering. The seed is ripe when the fluff fully opens. You see a creamy white or dark brown oval seed down to the fluff. They will be released and fly away. You can lose them, or tie a collection bag around them. You may also when half the seeds are ripe, carefully pull out the plant and put in the greenhouse out off the wind. Well continue to give water. In her wish to survive she will (more) quickly ripen all the seeds. So more seeds mature equal and faster, and are easier harvested together. Many plants that feel their end coming soon are still trying to make descendants.

Selecting you do thought. When you use the first lettuce that’s bolting as a seed carrier then a property of weeds to form seed quickly is kept. You then select to quickly bolting!

The grass seed mixture that you can sweep along the hayloft you can use to sow (parts of) grassland.

Good tomato seeds are in a juicy ripe tomato. Pepper seeds sit in a nice ripe pepper (green is always immature, so you never harvest usable seeds here!).

Let seeds quietly (not too hard) dry in the shade, where they cannot be blown away. Provide clear labels. To the seed you cannot always (exactly) see what kind of plant it is.

An annual plant completes its life cycle from germination to seed within a year. Examples are wheat and hemp. You have to sow them annually.
Biennial plants are monocarpic, which means that they bloom only once and bear fruit. In the first year, stem, leaves and roots grow. In the second year the plant blooms and produces seed, then the plant dies. So do not root crop seed plants after 1 year.
Biennials (leek, madder..) are usually sown in the late spring or early summer, then pricked and planted in the fall. They bloom in the following year.

There are also perennials (more year cycle).