How do I wire a one lamp fixture with a two lamp ballast?
The first question would be - "What type of ballast do you have?" If you have an old style magnetic ballast or electronic ballast. Then you will not get the single light to work properly. If you have a new style electronic smart ballast, it has the ability to sense how many lights are connected to it and adjusts to the load and will work (this is a very laymen explanation). You simply connect up the lamp sockets (tombstone) with the red and a blue and wire nut off the other blue. Black and white wires are you line in (probably 120V) to the ballast. If you are still confused there is a wiring diagram on the ballast which can help you.

Good luck.

PS Make sure when you do this the power is off. If they are newer fixtures you may have a ballast disconnect it's probably orange.