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You can just snip the onion flower bud off. This does the least amount of damage to the plant without affecting its ability to take in energy from the sun.

Yes, that will work. This article will help:
https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onion-seed.htm

No, you do not have to cut the onion leaves. This is how it builds energy to grow the onion bulb.

When the tops have yellowed and fallen over, they will be ready for harvest.

Harvest mature onions when their tops turn yellow and begin to topple. Loosen the soil around each bulb and pull or dig it up carefully to prevent damaging the bulbs. This article will help with storing: https://www.gardeningknowhow.com/edible/vegetables/onion/how-to-store-onion-sets-storing-onions-for-planting.htm

When you grow onions in your garden leave them in the ground until a pod grows on the tip of the green inside the pod are the seeds this takes a little while, hope this is what you were looking for...