Depends which way you want to go. Connecting an unbalanced source to a balanced input is straightforward. Ground one of the two balanced input connections, the other is now an unbalanced input. You may have a level mismatch however, standards are always a bit loose in audio equipment, but generally balanced levels are higher than unbalanced.
Going the other way depends on the design of the balanced driver. In the good old days when everyone used transformers to balance a line, simply grounding one of the outputs would make the other an unbalanced output. These days where differential drivers are used, you can just ignore one of the outputs, and treat the other as unbalanced. You might have to experiment to find which works. A word of caution, differential drivers don't like being shorted to earth. You could use a 110ohm resistor while you experiment.
The best way to do it is to use an audio balun - specifically designed for the task. Have a look here:
http://www.mmstudioz.com/Products.html
Never used these guys, but it explains the principal.