It is usually necessary to dry and clean the seed after harvesting. For seeds in pods, heads or capsules, a week or two at normal room temperature and humidity will dry them sufficiently. Once dry, the vessels will crack or split open and spill their contents. You can gently roll a rolling pin over stubborn pods to open them.
You can separate the seed from the chaff (dry coverings and other debris) by shaking it through a screen or kitchen sieve. A good trick is to place the seed and chaff in the crease of a folded piece of heavy paper or poster board, Tilt the paper and lightly shake or tap it. The seed will roll off the chaff and onto the table.
If you collect fruits or berries—for example tomatoes, blueberries or hollies—clean the seed of pulp before you store it. It may be easiest to mash fleshy fruits on a screen with holes smaller than the seeds. Rinse water through the screen to wash away the pulp and skins. Tough or unripe fruit can be left in a cup of water to ferment for a week to make the pulp easier to wash away.
Mash blueberries and other very small-seeded fruit on a paper towel. Once the towel dries you can scrape the seeds off with your fingernail. This works well for tomatoes.