It kind of depends on what you're wanting to do.
If you want a separate aux battery that you can run down without worrying about killing the main battery;
You can buy a battery isolator, that gets wired in between the alternator and the stock battery.
The unit will have two outputs, one to the stock battery, one to the aux battery. It will choose which battery to charge based on voltage and priority. I think some of these units end up with a .5V drop to the batteries but overall work well. Anything wired to the aux battery needs to have it's own separate fuses, wiring etc.
Another option is a simple relay setup. Use a high amperage charging relay coming from the main battery, then to the aux battery. Wire the relay so that the circuit is only open when the ignition is on.
If you just want to run another battery parallel with the main battery for more battery power;
It is pretty simple. Mount another battery, run the hot side of the batteries together, and the cold side of them together. Basically the same as you would when you are just starting a car.
But when you run a dual battery setup, both batteries should be new. If you use a brand new battery with an old battery, that new battery will only become as good as the old one that is used.
You can mount the aux battery anywhere you want, but it should be as close to the alternator as you can. The further away it is, the more loss you have in the cables. Cables should also be as big as practical to minimize loss.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT