When you put two battery holders in series, they add together to make one larger voltage. For instance, if you put a 4 x AA (6V) in series with another 4 x AA (6V) battery holder, then you are left with a combined value of 12V. If you were then to put a 2 x AA (3V) battery holder in series with both of those, it will add up to 15V.
Another quick tip is that you can bridge any of the battery compartments with a wire and decrease the value of the battery holder by 1.5V. For instance, bridge one of the compartment in a 2 x AA (6V) battery holder with a wire will convert it to a 4.5V battery holder.
You may be looking at my pictures and saying the values don't add up. Keep in mind that new batteries have slightly more voltage than they are rated for. When you put many in series, this surplus starts to add up significantly. For this reason, if an exact voltage is important in your circuit, always use a voltage regulator. If you decide to use a voltage regulator, make sure you have a power supply rated for a few volts more than the regulator, or you run the risk of under-powering it.