Shannon and I always plant onions in our garden. Shannon loves onions, and I do too as long as they are cooked, so we go through quite a few of them throughout the year.
We always buy some sets in the spring and plant them then harvest the green shoots all summer long to use as green onions before harvesting the onions themselves near the end of the season.
If you don't trim your onions they will flower, which we didn't want them to do (except at the end of the season), so we trimmed them pretty regularly. We used what we could fresh, but of course there was way too much and we ended up dehydrating quite a bit. Dried green onion is great, and even though we used it regularly we still had enough from last year to last all the way to planting season this year!
To dehydrate your green onions you simply cut them into little pieces (scissors are quicker and you can cut many of them at once - only one is shown in the picture to make sure the picture was clear) and then spread them out on your dehydrator tray (the without holes), set it to the proper temperature, and turn it on. You can also sun dry them. Just put them in a cake pan, set them outside in the sun, bring them in at night, and stir them several times each day. By the end of day two they should be dry. 8 cups of thinly sliced green onions equals almost 1 cup dried.
If you are going to dry them in your oven, be prepared....you house will smell like onions for weeks! If you have to dry them in your oven, slice the onions, spread on a tray/cookie sheet/cake pan, put in the oven, prop the door open about an inch an set your oven temp to the lowest setting (150 or lower) stir often. This will take 2-3 days, or longer depending on how big you slice the onions, how often you stir them, and how thick you put them in the pan.