Take the device that you wish to convert to AC from DC with you to an electronics store that carries a wide range of electronic components and accessories. Stores that sell big ticket item such as TVs and computers generally have less varieties of power inverters than stores that the focus on electronic accessories (see Resources below).
Purchase a power inverter that can handle the power requirements of your device. Check that the voltage on your device matches the voltage on the power inverter. Also check that the amps required to power your device does not exceed the amp rating on the inverter.
Plug the male end of the inverter into your vehicle's lighter outlet, then plug your device into the opposite end of the power inverter. Set any power switches on the inverter to the "on" position, then power on your device.
