Since broccoli is insect pollinated, the crop has to be isolated from most of the other common brassicas that may be in flower. Pollinating insects do get around (they can travel and move pollen around over many hundreds, perhaps thousands of feet) and if other members of the broccoli species are in flower at the same time, they will be crossed. Cabbage, cauliflower, kohlrabi, some kinds of kale, brussels sprouts are some members of the Brassica oleracea species. Breeders have crossed broccoli with cauliflower with some interesting and marketable developments, but for the most part you will probably want to grow only one of the Brassicas at a time, and only one cultivar if you want to maintain seed purity.
Once broccoli starts to flower, it bolts quickly and attracts many kinds of insect pollinators. We get an equal number of bumble bee and honey bees working the flowers as well as flies and cabbage butterflies. Rarely, broccoli can self-pollinate but mostly it doesn't. Sometimes pollination is hit or miss and seed production can be erratic. Pollen will generally have to be transferred among several plants. This self-incompatibility may help to facilitate chance hybrids that show greater vigor.
Pollinated flowers develop siliques or mustard type seedpods on elongating flower stalks. When most of the pods enlarge with seeds and turn brown pull up the plant, roots and all, and allow the plant to dry in a well ventilated area until it's pods are brittle. Cut off the roots and the thrash the stalks (roll the seed pods back and forth through your hands until the sides of the pods break free and seed is released. Use a strainer or screen to separate seed from pod remains. Finally, gently blow away fine debris with a fan to leave the clean seed behind. I had some difficulty planting the OSU broccoli early enough to obtain the best quality seed. Because of the density of the pods on a single plant and the fact that seeds develop on one broccoli plant over an extended period of time, damp, wet weather can play havoc with the seeds that mature early on as you wait for the later pods to ripen. Seed harvesting in the wetter part of late summer is just not conducive to producing quality clean seed with high germination. Fortunately, this year the seed crop is coming along just fine and hopefully will ripen in the field before the late summer damp weather. For small-scale seed production, good quality broccoli seed should be fairly easy to produce on the farm but dry conditions as the seed ripens in the pods are really important. I was hoping to put up a polyhouse for drying the seed crop out during damp weather but that remains a task for the future. When I was in the seed business I always hot water treated my brassica seed before packing; 122° F for 25 minutes, cooled and quickly blown dry (without heat on screens) just to make sure that I wasn't going to distribute any seed-borne fungus diseases. I never do that for my own seed and I haven't had any major disease problems. So you decide.
For brassica seed (that includes broccoli, turnip, collards and many other vegetables) the dry pods are crushed to release their seed and then cleaned. In small batches which is what we work with at the farm, the seeds get separated with assorted strainers and screens. The work of separating the seed from chaff at the end of the process requires a fan or air compressor (or in this case lung power).