How to Set a custom white balance on the Canon 5D Mark II

Most DSLR's like the Canon 5D Mark II come with pre-programmed white balance settings: daylight, cloudy, tungsten, fluorescent. But what do you do when you're in a mixed lighting situation and you want to balance between exterior daylight and interior tungsten light? If you set it to daylight, your tungsten light will be too orange. If you set it to tungsten, your exterior light will be too blue. The solution: set a custom white balance to balance the disparate lighting situations.