I'll add this in just to save you time in case you are moving to a DOULBLE set up with 4 6V batteries.
We'll call the batteries 1/2/3/4 from left to right. You essentially have 2 12 volt batteries wired parallel to provide you with 12v but double the amp hours of a single pair.
In this case...you can attach external devices directly to the + terminals of EITHER or BOTH battery #2 and battery #4. All fuses should be attached to those two terminals as well.
Negative wires may be attached to the negative terminals of Batteries 1 or 3...or to the negative bus found at the other end of the wire already shown connected to battery #1.
Note that the Way the batteries are lined up is not material as many people put them in a square or have each pair a little away from each other based on space needs. What matters in all cases is for you to figure out the wiring between the two pairs to match the WIRING in the diagram above.