In your zone you absolutely need to start them indoors. I would suggest starting around February in your zone for a mid-late April plant out. Give them a good potting mix and nitrogen once you see some green top growth.

Seedlings should be growing in about 4" of soil, 6" would be OK. The seed is lightly scattered on top of the soil mix and 'raked' in just enough for good contact. Keep moist the entire time.

Once they sprout they need light so a floro light or very sunny windowsill will be needed for good bulb growth. You want the bulb to be about the width of a pencil or more when it is time to plant out.

You can kind of think of young onions (we are talking about onions?) like grass. Light, nitrogen and water.