It's actually quite simple. For a dual battery set up with the batteries switched (that should be how you plan to do this)
To add a second, third, fourth, etc battery to an existing circuit. Hook the batteries in parallel. Leave the negative side of your current battery connected as it is and add a cable to the negative post(s) of the new battery(ies).
For the positive side, take the positive cable off the current battery and install it on the battery switch center terminal, or output terminal. Install a new battery cable on each battery going from the positive battery post to the switch batt1 and batt2 terminals respectively. Make sure the battery switch is well over the high water mark and preferrably in a compartment other than that of the engine.
If you are unsure of the difference between series and parallel circuitry, give me a call I and I can help you.