Had one in my old car,
Put the battery in the boot (if thats where its gonna go), Earth the negative of the battery well to the car chasis. You can then run a 4 gauge wire from the positive to the back battery.
But you will need to put a split charger relay on the link as if kept connected whilst the car is not running the two batterys will discharge each other. The cheapest one I found was at maplins.
For something like £15. you will need to connect the relay to the ignition, which activate the relay once car is running and charge the back battery. Or you could just have a switch but you run the risk of forgetting it switched on!!