1. Look for seeds from plants which have particular characteristics that you like. For example, you might collect seeds from plants that stayed small, because they will need less water to grow. Or you might choose plants with multiple seed heads, or plants whose pods or grain heads are large. You might also look for plants that suffered less pest damage than others. Seeds from these plants are likely to produce more plants the next season, which retain the same qualities.
2. Choose seeds from plants that are different from each other. Pick seeds from plants of different colors, sizes, and with differently-shaped leaves. Diversity breeds healthy genetics.
3. Select some seed from each area of the field, and from a many different plants. Just pick whatever seeds you come across, at random.

Random Seed Collection

Having a wide variety of seeds will help acquire a good collection of seeds with different qualities. Not all plant differences are visible to the naked eye. Some of the diversity which helps plants resist pests and diseases or adapt to changes in climate is contained in the seed itself. That is why you should collect some seed at random. The random sample ensures you are getting a good mix of different qualities, including ones which you cannot see.