I superglued a two-pin .1" female header to the side of a coin-cell battery holder and soldered wires from the holder's terminals to the header's pins. I also added another header with the metal clips removed to the other side to help stability. This slides onto the Galileo battery pins and sits nicely on top of the board.
Be sure you get +/- right. I don't know if the board has any protection against reverse voltage.
Here are the parts that it took:
Add a CR2032 battery that you can get at many stores or online for cheap.