Here's what we do:
1. Buy green onions from the grocery.
2. Use the greens for what you need, cutting and saving the bulb/roots part of the onion.
3. Put the bulb/roots part of the onion in water. It will actually start growing roots, and growing from there.
4. Plant bulb/roots into a pot with some soil.
5. Cut whenever you need green onions.
Okay, so technically you can skip 3 and it'll work out just fine. But, it's kinda fun watching all the roots grow. It's good affirmation that the process will work. :-D
Here are a few pictures:
These last two pictures are the ones that we've had for at least 3 months and have been trimming for use when needed.