Heirloom plants are older varieties of common plants such as tomatoes, beans, corn and others. Hybrid varieties are created by combining the genetic material from two similar plants, such as two different types of tomato, to create a new plant that has the best traits of its parents. But hybrid seeds don't reproduce "true to type," so if you grow them you might end up with an undesirable tomato or other plant. Many heirloom plants have vanished in today's world of hybridized plants, so if you cherish the past, you can save your heirloom seeds and plant them later.
Purchase seeds that you know are heirlooms to make certain their seeds will grow into the same plant that they came from. Heirloom seeds are also called "open pollinated" and are available from specialized seed companies.
Plant and grow your heirloom seeds as you would any other seeds--follow label instructions for soil type, depth of planting, watering requirements, fertilizing and so forth. For annual vegetables such as tomatoes, wait until late summer and then allow one or more fruits to ripen on the plant longer than you would if you were going to eat it. Don't allow the fruit to rot, but let it grow large and plump.
Cut open the fruit, or gather seeds from dry flower heads. Empty seeds into a sieve small enough that it won't let any seeds slip through and then hold it under running water for a minute or so. Rub the wet seeds between your palms to remove any fruit that sticks to them. You might need to gently remove fruit pulp, such as the flesh that adheres to pumpkin seeds, with a soft brush--a toothbrush works well.
Spread seeds on a window screen or other type of screen that has tiny holes. Place the screen in a dark, dry, well-ventilated area such as your garage. Prop the screen containing your seeds on top of several bricks or boards to give the seeds good air circulation, which they need to dry properly.
Store your dried seeds in plastic zipper bags or jars that you seal with two-part lids. Keep seeds in a cool, dark place. You can store them in your refrigerator, but unless the seeds need cold stratification to make them germinate later, do not store them in your freezer. Lavender is an example of a plant that has seeds requiring cold stratification.
Label each bag or jar so you'll remember what's inside. When planting time comes the following season, plant your seeds as you would any others.