Depends if the battery is going to be permanently mounted to the car or if it's just a temporary arrangement and also how quick you need it to charge. In addition, if there's going to be load on it during charging. For a permanent installation I would run a decent, thick cable from the car battery through a fuse to the rear of the car, with a caravan split charge relay or similar to enable charge to the second battery when the car battery volts rise.
You want to drop as few volts in this cable as possible because if there's too much drop the battery won't reach a fully charged state. If you need to draw load from the battery during charging it might be wise to take this from the end of this cable at the front of the car to minimise voltage drop at the battery end.
For a temporary set up, something to plug into the cigar lighter may do the job, depending on the voltage at that socket with the other loads in the car drawing current. You might find the lighter socket delivers insufficient voltage (less than 13.8 and the battery won't reach full charge) to charge the battery. You may well find that the resistance in the circuit is enough to prevent the current getting excessive, since the battery "looks" like a voltage source of up to 12.5v with an internal resistance. This means that you're only dropping a volt or so across the resistance of the circuit instead of the full 12 volts - unless you start with a totally flat battery, of course.
If it's a temporary lashup and you've got an inverter and a battery charger handy it might be the easiest solution. At least you know the charge to the battery will be under control.
Kevin