My climate is likely a bit atypical, but here’s what I do that seems to work (my growing season is too short to mature many crops outside):
Seed gets started indoors ~Feb 1. Gets xplanted to the greenhouse after most have a couple of leaves and grows in the GH until roughly pencil diameter. Then gets xplanted again to the garden roughly end of April. At this point they are roughly the size of purchased onnion plants, and I clip off some of the tops to balance the root loss. Anyway this is my pattern for growing onions here.
For seed production, I try to save a few of the larger onions that are in storage from the previous fall that have desired traits (large, no disease, etc). We use up any that have started to sprout or go bad, so in effect we are selecting those out of the gene pool. Around March 1st I select a few of the best onions for planting for seed. Again in the GH (cause our season is not long enough to mature onion seed reliably). After a few weeks those onions start to grow, they generally will flower in August (give or take some). I let the seeds mature to almost the full dry stage on the plant. But try to cut and complete the drying inside, since fully dry onion flowers tend to shatter and scatter the seed. When dry I rub the flowers/seed between my hands to break apart the seeds from the dried parts, then winnow.