Take two of the batts and use one of the thick but short connecting cables from a positive of one to the negative of another. Now, just, in your mind, consider this a 12 volt batt, which it is, and forget about the connecting cable, it's not there as it is of no consequence which leaves one positive and one negative post available.
Now do the same with the other two half's of the other 12 volt batt so you end up with two 12 volt batts.
Now wire the two available negative post's, and two positive post's together, and in essence, you have a double sized 12 volt batt.
Next connect the cables that go someplace other than batt to batt, to an end positive or negative post (with the negative connection connected LAST), and you are done.
This information, along with the diagram should bring it home, I find that a large portion of people find it much easier once getting rid of the 6 volt mentality, and realize, mentally, a two part 12 volt battery.
I personally, run my rig with 24 volts, so my battery's, for convenience, come in four parts of six volts each, which after EXTERNALLY (vice internally) wire them together provide the 24 volts, much easier to handle than a one part battery!