Here's how you do it:
Early in the season, when the flower buds are strong but not yet blooming, place a poly "wedding favor bag" around a flower bunch prevent any cross pollination. If a flower sets fruit inside the bag—remember, tomatoes are perfect flowers and can fertilize themselves—remove the bag and place a twist tie or similar on the stem. That is the fruit you'll save seeds from, as you know it will be pure, without any crossing with any other variety.
Once the fruit ripens to dead-, soft-, almost-exploding-all-over-itself-ripe, remove it from the plant and bring it somewhere you can work. Cut the fruit open and slip the pockets of seed and gel into a small bowl or mug, scooping out as many seeds as you can. Feel free to slurp up any remaining tongues of flesh you have left over after scooping out seeds.
Add a splash of water to the small bowl of seeds and gel so you have at least a quarter cup of liquid. Set the bowl somewhere warm but not in the sun—in an unused part of your kitchen (yeah, right), or perhaps in a shed or shady, protected place. Let the bowl set and grow mold. Depending on the temperature and time of year, that floating layer of mold can take anywhere from a day to three. If it's hot, like it is here in the summers, add water if it looks like the bowl is drying out before mold forms. Why the heck do you need to do this? Each seed in a fresh tomato sits in a tiny purse of its own gel that contains a hormone preventing the tomato from germinating inside the warm, wet fruit while it's still on the plant. However, after falling to the ground, a tomato rots, and that rotting process frees the seed from it's protective pillow, allowing it to germinate when the weather, moisture, and warmth are right. When we save seeds ourselves, we've got to emulate the rotting process by using very ripe fruit, collecting seed, and letting it mold over.
If you live somewhere warm and dry, like we do, the seeds will be thoroughly dry in a week or ten days; somewhere else, you may need to wait longer for the seeds to completely dry.
Once dry, place them in a labeled container that works for you (small sealing ziplock bags, little snapping boxes screws come in, tiny jars), and place that container somewhere even-temperatured and dark, where the seeds can survive with decent viability for seven years.