I'm growing 2 varieties from SSE that were brought from India. The man lives in cool northern CA and cannot grow them out for seed in his climate. I only asked for one,but he sent 2 kinds and I would really like to continue them for him. He wrote such a nice letter telling about the little village that he visited and the hospitality of the people.
"You want to save seeds from your eggplant or squash. The seeds from either are immature when the fruit is ripe enough for you to eat. Collect your seed from these mature oversize seed pods after they have been left in the garden fully a month after you have cleaned up in preparation for winter. Nature considers these seeds to be ripe about the time their containers have been destroyed by the weather. Note that there is minimum of thirty days difference between deadheading flowers to promote additional flowering and cutting flower heads for seed collection."
Let the fruits you want seed from to really ripen. They should look dull, not shiny when it is time to take the seeds. Than cut them open and pop the seeds out. The seeds need to be light brown to be viable. Dry on a paper for a day, like peppers and than store.
Marshseed will probably post there own response but since I a food processor I'll toss my hat in the ring.
Scoop the pulp into the food processor, add maybe twice the volume in water. Pulse or puree till you have a slurry. Pour into a jar or some such or just leave it sit in the processor jar, the seeds settle to the bottom the puree floats to the top. Pour off puree, rinse a few times like (fermented) tomato seeds. You will probably chop up a few but there are SO many seeds per fruit and you're using more than one fruit (hopefully) so you'll still have a gazillion seeds when finished.
My blender is too heavy duty for this, hence the processor but the regular blender for making milk shakes or whatever (not the grain grinding kind :), should work well too. Maybe pulse a few times and see how much damage you're doing, then go for it.