It's not rocket science...but it's fun...and informative...and I just did some of these last weekend and thought you'd want to know, and see. :)
Start by slicing up some onions. Try not to cry...although they always make me tear up. Just like chick flicks! :)
Arrange the slices in your dehydrator however you'd like. If you are doing several onions you'll probably want to keep those slices from the first pic together. If you're just doing one onion like I did this time, you can separate them out so they'll dry more quickly. Whatever floats your boat! :)
Turn on your dehydrator (away from any drafts and anything flammable of course!) and let it do its magic! Then check on these little guys every 6 hours or so. Mine only took 12 hours to completely dehydrate, but then I let them cool down overnight before storing them.
Once they are dehydrated you can either keep them as slices or chop them up like below. I wanted the versatility of having diced onions on hand (because they are SO handy!) so I chopped mine all up. Then just store them in an airtight container at room temp until ready to use. You could also freeze them...but since they are dehydrated that'd be kind of overkill don't you think? :)
Happy preserving and dehydrating! :)