A hobby involving batteries is a great way to learn about the physical properties of electricity. Whether you're toying around with lights, building a circuit or trying to create a battery-powered backup, understanding how a battery works is a valuable skill. Creating a battery "series" is a method of lining up batteries of a certain voltage...
M/F Lay out the batteries so the positive end of one battery is next to the negative end of the other battery. The opposing polarities should be facing each other on the inside and facing outward on the outside.
M/F Tape a copper wire connecting the positive side of battery 1 to the negative side of battery 2. Alternately, you can solder the wires to the batteries for a more elegant and long-lasting option.
M/F Attach a wire to the unconnected side of each battery, creating two wires that are connected at one side and unconnected at the other. The wire coming from battery 1 is a negative polarity, while the wire from battery 2 is positive.
M/F Connect these wires to the appropriate hookups on the device you wish to power. There should now be 24 volts flowing through the system.