1. You’re already growing the seed bearing part. (Those are those bits inside the ripe fruit.) Unlike onions or arugula or most other veggie crops, there are no extra steps to get to your crops to go to flower and produce mature seed.
2. Tomatoes are selfers (remember last post?) so they barely cross. It’s ok to ignore isolation distances (as long as you’re not selling tomato seed or you’re not one of the only stewards for a specific tomato variety) You’ll barely see any crossing.
3. You don’t need that much tomato seed in a given year. Saving seed from 10 to 20 fruits gives you 1000s of seeds – that might be enough seed for 3-5 years of plants.
p.s. Make sure you’re saving seed from open pollinated tomatoes (OPs) and not hybrid seed (F1). Most seed catalogs will tell you if the seed is F1.