Onion power is very easy to make, although, can be time consuming if you don't have modern equipment. You simply take one or more onions and peel them. After peeling the onion I chop it up into very small pieces. I then put my pieces in a food dehydrator. If you don't have a dehydrator, you can put them on a cookie sheet, spreading out the pieces, and place them in an oven on about 150F degrees. You will know they are ready when they crumble in your hand. After I dry the onion pieces, I put mine in a food processor. You can use a coffee grinder, pepper grinder or any type of grinding machine. You can also do this with a rolling pin or some type of crushing utensil. You grind the onion until it is the consistency you want it. You must have some type of air tight container to in which to store the powder.
Hope this helps.