It's pretty easy with your A700. You'll see a dedicated WB button on your camera. Just press it and scroll down past the presets using your controller pad until you get to the Custom choice, then scroll right until you see the Set feature. If you select that (just use the center button on the controller pad), it tells you to use spot metering.
If you're using a gray card (anything white can work, too), just place it in between some of the lights on the floor (since sometimes they'll vary in temperature and you're trying to get a good average), fill the frame with it (being careful not to block the lighting), and press the shutter button and the camera will measure the temperature of the lighting from the card. It will then ask you where to save it. You've got 3 memory locations for custom white balance settings with the A700. Just pick one of them (Custom 1, Custom 2, Custom 3) to store it. Then, you can easily select from one of those custom readings as desired by pressing the WB button again. That allows you to store different settings for different locations (or lenses, since if you swap lenses, you'll sometimes see variations in WB between them using a custom setting).
Since you've already got an Expodisc, I'd just use it instead. Basically, you point the camera towards the lighting you want to shoot in and set it the same way you would from a gray card. The Expodisc is designed to diffuse the light so it's more even and lets the camera measure the temperature of the lighting in the same way.
Note that one popular way to set it was using a pringles lid in the same way as the Expodisc, as they used to be a translucent gray. But, the newer lids won't work (they're too clear for that purpose). LOL