How do you wire a 12 volt?

Wire a 12 volt circuit in a loop: battery positive to an inline fuse, fuse to a switch, switch to the load, and the load's other side back to battery negative. The fuse sits as close to the battery as physically possible, because it protects the wire, not the device.

Switches always break the positive side. For a 3-wire lighted switch, power lands on the input terminal, the load on the output, and the third terminal goes to ground so the indicator can light.

Crimped ring or spade terminals beat twisted wire everywhere on DC.

Wire gauge matters more at 12 volts than people expect; at low voltage every lost volt is a bigger fraction.

The DC circuit calculator sizes the wire and the fuse together for whatever you are feeding.