Voltage is used when referring to electricity. Devices that require electricity have a written note that indicates the voltage required and whether it's direct current (DC) or alternate current (AC). Most of the times, devices come with adapters that allow you to plug a 12 volt machine in a 220 volt system. When using different devices, they must all be the same voltage.
Check if the device operates with direct current or alternate current. Note the current type.
Convert a 12 volt AC to 24 volt AC by simply plugging the transformer to the 12 volt device.
Switch from 12 volt AC to 24 volt DC. To do this, plug the step up transformer directly to the 12 volt device and then use a rectifier bridge to make the current flow as DC.
Change from 12 volt DC to 24 volt AC by using the switching converter and then the step up transformer.
Convert 12 volt DC to 24 volt DC. Use the switching converter to turn it to AC, then the step up transformer. Finally, use a rectifier bridge to bring it back to DC.