Let the bell pepper plants that you desire to harvest the seeds from fully mature. Don't pick the fruit from the vine until just before frost. Most green peppers that you buy in the store or grow and eat at home contain seeds that are too immature to be viable as stock seed for next year.
Pull the plants from the ground and shake off the dirt. Hang these plants with fruit still attached either in a basement or out building. You can string them up to the rafters or beams. Make sure that it is dry.
Wait until the plants have completely dried and the fruit has shriveled up and then pick them from the plants. Open the fruit and separate the seeds.
Place the seeds on a plate to further dry for about another few days and then package and store in the freezer until next spring. Be sure to label the package with what they are and when they were harvested.