Just a few tips on an easy way to save tomato seed.
Pick the biggest, best, healthiest outdoor tomato you can find and make sure it has stayed on the plant to really ripen properly.
Squeeze the seed out into an egg cup and add just a couple of teaspoons of water. Leave this eggcup on a warm windowsill in the kitchen for several days. The tomato pulp surrounding the seed will start to ferment and break down a little, allowing you to separate it from the seed more easily. Don't worry even if a little mould appears on the surface of the pulp, that is fine, so long as the tomato flesh starts to decay it is ready.
Gently wash the seeds in a sieve and rub with your fingers to separate most of the surrounding pulp from the seeds. You won't get it all, but get as much as you can and this will prevent mould forming on the dried seeds through the Winter.
Spread out the seeds on a plate and separate them. When they are completely dry you might have to scrape them off the plate to store them in packets.