Here's how to do it.
First, peel the ginger completely.
I used a peeler, but later learned that a kitchen spoon does a better job getting into the nooks and crannies.
Put it in a freezer bag; press the air out before sealing. Write the date on the bag.
When you want ginger, remove the bag from the freezer; take the ginger out of the plastic wrap. Grate the ginger, using a microplane grater. Return the unused portion of ginger to the freezer, as before.