When are onions ready to pull and store? Rule of thumb is when 2/3 of the tops have fallen over. You can bend over the rest of the crop. Also, stop watering. Leave the onions in the ground for a couple of weeks.
Pull or dig the onions out of the ground not removing the tops. Do not clean the bulbs. Take the onions into a shady spot like a garage or under a tree without crowding them. I place the onions on an old window screen sitting on blocks so the underneath has air circulation. Leave until the leaves are brown and brittle.
Now you can remove the tops and roots but leave the dry skins alone. Store in a dark, dry, and airy area. Use milk crates, burlap sacks, or mesh bags. How long they will store depends mainly on the variety. “Sweet onions” do not last very long. The variety I plant is “Copra” which lasts an easy 6 months.
The onions I planted were ground for seed started 10 weeks before transplanting outside. I found the onions growing from seeds do not send up flower stalks like the onions planted from sets.