how to make crystallized ginger powder
The normal way of making crystallised ginger is to boil 1-cm cubes or slices of ginger briefly in water a couple of times to get rid of the bitterness, then simmer it for a long time in syrup. Once it's nice and soft, you drain off as much of the syrup as possible, then coat the pieces in granulated sugar. 

If you have already grated it, I guess you could still use the same method, but you wouldn't need to simmer it in syrup for so long, and draining the syrup off might be a challenge. If you add enough sugar, it will eventually absorb the remaining syrup, and I am sure that the surplus (gingery) sugar won't go to waste!