Locate the positive and negative terminals on the batteries. Some batteries have a "+" mark to identify the positive terminal and a "-" mark to identify the negative terminal. Rotate the two 6-volt batteries so that the positive terminal of one battery is facing the negative terminal of the other battery.
Connect the two batteries in series by connecting the positive terminal on one battery to the negative terminal on the other battery. The connection can be made using an electrical conductor with a connector that fits the battery terminals. Different connectors are used for different batteries sizes and types.
Connect another electrical conductor to the negative terminal of the first battery. The other end of the conductor is then connected to serve a load to the appliance requiring voltage. Repeat this step with the positive terminal on the other battery.
Test the connection with a voltmeter. Connect the black lead of the voltmeter to the wire that is connected to the negative terminal on the first battery. Connect the red lead to the wire that is connected to the positive terminal of the second battery. The voltmeter should read 12 volts direct current (DC).