Juice half of the lemon or lime into a glass. Save the other half for later, or use it for a friend. :)
Grate some ginger root into the same glass.
Fill the glass to the top with cold soda water.
Enjoy served cold!
I leave the grated ginger in the glass, but you can juice some of the ginger, and strain out the pulp if you prefer. I do use a ginger grater which helps juice the ginger, and doesn't leave as much pulp behind. (I've linked to a similar grater below, one with what I believe is a better design than the one I bought.)