Q.Turnip greens that have gone to seed
After the yellow blooms with seed, when do you gather the seeds to replant later?

The seeds are mature and ready to harvest when the pods are dry. Harvest the individual pods or the entire stalks.
Place the seed pods in paper bags to help from loosing the seeds.
If your growing more then 1 variety, the plants can cross pollinate and not be true to seed. 1/4 mile distance between varieties is recommended.
https://www.gardeningknowhow.com/edible/vegetables/turnip/bolting-of-turnips-plants.htm
https://www.gardeningknowhow.com/edible/vegetables/turnip/turnips-growing.htm