You don't say what kind of camera you have. The custom white balance process differs between brands.
I carry a white-balance reference in my back pocket. It's a plastic card, 4" x 6" and about 3mm thick so it's sturdy. It made by RMI and it's called the Digital Gray Card (although it's not sold for exposure...only for white balance.) I have a Nikon and setting a custom white balance is very easy. I just hold down my WB button for two seconds, whip out the card and face it toward the primary light source, frame the card in the camera, and press the shutter...done. It only takes a few seconds, so I do it any time I step into new light.
I find that a custom white balance always looks better than one of the standard settings, and better than trying to set it myself in post-processing. The eye is a fairly poor calibration tool...if it was a good calibration tool then we wouldn't need to buy hardware to calibrate our monitors.
Here's a link to the gray card I used...
Digital Gray Card