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While I use a different brand of DC/DC converter, you will need two fuses - one for input (48V) and one for output (12V). I run the input and output with a double pole relay with a 12v coil. Both input and output are normally open. When I turn the key, power goes to the relay coil, turning on the converter (48v) and powering the 12v system (and charging the battery). If you leave it hooked up to the 12v battery, it will probably backfeed about 50 milliamps or so, draining your aux battery when you aren't driving the car.
Depending on what type of aux battery you have, you will need to match the converter output to about 13.6v (for flooded) or about 13.3v (for sealed) Check manufacturer recommendations for your specific battery.
If you have the voltage too low, it won't charge properly, and too high will fry a sealed battery (flooded are more forgiving).
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