Simply peel your onions, and chop them up. You can use a non-electric hand chopper or simply dice them up quickly in a food processor. You can also dry them in slices if that's what you need for future meals, but they are too large to fit into a coffee grinder to make into powder.
Spread the chopped onions on parchment paper lined dehydrator trays, and dry in your dehydrator according to the instructions it comes with.
When the onions are dried, simply break apart your diced onions for "flaked" onions. Place the remainder in a small coffee grinder and grind until it becomes a powder.
Store the onion flakes in airtight glass jars in your spice cabinet. These will also collect moisture, so you may want to store them using my next tip.
Store the onion powder in your freezer to keep it dry and prevent it from hardening. If you place it in your cabinet it will quickly harden, unlike garlic powder. You can also put some white rice, a few beans in it, but I typically don't shake mine out. I measure with a measuring spoon, and it's much easier for me if I don't have anything in the onion powder. Shallot powder also hardens over time.
I recycled a jar the size of a half pint to store my onion powder, using a screw top lid. My canning funnel fits in it perfectly to pour it out of the grinder easily.