How do you wire two 12 volt car batteries to make one 24 volt battery?

To use two 12 volt batteries in a 24 volt system they must be wired in series. For this explanation I will refer to them as battery #1 and battery #2.

This answer describes the wire connections for a vehicle with a negative ground electrical system.

The cable from battery#1 Negative terminal connects to ground (Frame or engine block).

The cable from battery#1 Positive terminal connects to Battery #2 Negative terminal.

The cable from battery#2 Positive terminal connects to Starter solenoid.