1. Plant at least six or more plants and save tomatoes from the best plants. Even though tomatoes are inbreeding and typically self-pollinate, they have many polygenic characteristics that may drift over time if there is no selection pressure. Save seeds from several tomatoes just in case that one tomato was crossed you won't lose the variety. You should select for yield, quality, suitability, and horizontal resistance as described in Raoul Robinson's book Return to Resistance. Don't save seed from defective or odd plants.
2. When you are harvesting ripe tomatoes take some of the pulp, juice, and seeds from the selected plants by squirt, squeezing, or scooping it into a sealable sandwich bag.
3. Close the bag and leave it in the cupboard for 3-5 days to ferment (rot) to kill off any pathogens that may be present in the fruit like Tobacco Mosaic Virus. This fermentation and rotting is similar to the natural process that occurs in the wild. Don't forget about it indefinitely...you know what I mean.
4. After three days fill the bag with water, pour of the pulp, the good seeds should sink (don't pour out the seeds). Any unfertilized seeds should float and get poured out with the pulp. If you lose a few in the floating pulp, that is probably okay. You should have more seeds than you can plant. Refill the bag with water and pour off the remaining pulp a few more times. There should just be a bit of water remaining on the seeds.
6. Invert the plastic bag, dumping the wet seeds onto a glass or plastic plate to dry (don't put them on paper towels or napkins as they will dry to them like glue). Carefully pour or blot off any extra water to speed drying. Put the plate in an empty cupboard or cabinet out of the way.
7. Let them dry for at least a week and then test the dryness by breaking one of the seeds if they dry and show not evidence of dampness you can dry them for longer or put them into bead bags or old glass bottles for next year.