Here is a pictorial from google images on how two 12VDC batteries are connected in parallel or two 6VDC golf cart batteries connected in series.
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What I do here alot if I have removed my battery terminals for one reason or another is the find the word 'NEG' or symbol '-' stamped into the battery case and make sure this is the where the frame ground cable is going to.
It is very easy to get the cables mixed up and if that happens it immediately blow a in-line fuse close to the battery bank as well as two fuses back on the 12VDC Distribution Panel MARKED REVERSE POLARITY.
If you find these three fuses blown be sure to correct the bad wiring first before replacing the blown fuses as it will just blow them again...
Roy Ken