To use the ExpoDisc is pretty simple,
1. Put the ExpoDisc on your lens.
2. Set your focus mode to manual.
3. Take a picture from a well lit place in the scene where you are about to shoot
4. Set your camera to custom white balance
5. Select the image you just took took as the white balance reference
6. Set your focus mode back to Auto if you want
7. Use this white balance for all other shots in that place
Now here is a trick to achieve very good white balance results just the same way but free 🙂 all you have to do is replace your ExpoDisc by a pringles can cover and use it the exact same way specified above, it will virtually work as well as the ExpoDisc.
I know there are many professional photographers out there that are going kill me for saying that the results are as good as the ExpoDisc, but for an amateur like me the results are more than satisfactory and will save me some bucks.
I read this trick somewhere on the web a few weeks ago
Here is another tips, if you prefer the white card manner you can also use the grey cover of the film protectors, it also works as a charm.