http://www.aquariacentral.com/forums/showthread.php?t=147012 All ther is to it is just snipping a few wires, adding a 2 way light bulb socket, and tying the wires together. Then screw the sockets into the fixture so they don't dangle and add the light bulbs. Make sure they fit first, or else you will come home to bulbs that don't fit.
BTW, here are the bulbs you should try to find:
With 2 of the 15 watt lights over a 15 gallon tank, you will get 2 WPG, enough for most aquarium plants out there.