Inspect the petunia plants after the flowers have faded and the petals begin to fall off. Locate the seed pod, which is an oblong, swollen pod produced where the flower was located. Immature pods are green in color.
Cut the seed pod from the plant with shears once it turns tan or brown, but before the pod begins to split open. Place the pod in paper bag, which catches any seed that may fall out from the pod.
Set the bag in a warm, well-ventilated room for seven to 10 days so the pods can finish drying. Leave the bag open to allow air circulation around the pods.
Hold the pods over a bowl and split them open with your fingernail. Shake the seeds out of the pod and into the bowl, separating any plant material from the seeds.
Place the seeds in an envelope labeled with the petunia variety or color and the harvest year. Seal the envelope closed and store it in a cool, dark place until you are ready to plant the seeds.