You can harvest some to use green otherwise cut off the seed stalks so they can put the energy into forming the bulbs. Many people harvest a little bit all summer to use then harvest the rest in the fall to store for winter.

If you haven't had /used green garlic - it looks like and is used like green onions (at this state it doesn't have a bulb)

If you only want bulbs, then cut the seed stalks and leave them. You can dig one to see how they are forming.