Before you go through the trouble of harvesting and planting the seeds from your tomato plant, get to know their history. If they are hybrid varieties, then their seed is likely to be infertile or grow into a plant that shares few if any of the characteristics of its parent plant. Only heirloom tomato seed can be planted reliably. And if you take the time to harvest and plant your seeds with care, you can produce unlimited generations of new plants.
Pick a ripe tomato (firm with full color) when the summer average temperature is around 75 degrees Fahrenheit.
Cut the tomato in half horizontally, scoop out the seeds and surrounding gel and deposit them into a glass jar to which a cup of water has been added. Leave the seeds in the jar to ferment for five days. Stir the mixture twice daily.
Pour off the matter and seeds that have floated to the top. Scoop out the viable seeds in the bottom of the jar and rinse them off.
Spread the tomato seeds out on a paper towel and allow them to dry for 24 hours.
Place the seeds in a sealed glass jar and store it in the refrigerator until early spring or three weeks before the last predicted frost date in your area.
Fill a 6-inch pot to within 1/2 inch of its lip with a potting soil mixture that is one part potting soil, one part perlite, one part sphagnum peat moss and one part compost.
Plant the tomato seed 1/8 to 1/4 inch deep and water the pot until you see water dripping out of the drainage holes in its bottom.
Cover the top of the pot with plastic wrap and place it in a warm, sunny spot.
Remove the plastic wrap in three days. Continue to keep the soil moist with regular watering until the tomato seed germinates in one to two weeks. One week after the seedling germinates, allow the top inch or so of the soil to dry out before watering.