sharif_al-
That just means expose so that the white object - a piece of plain paper or whatever - is white. Generally, that's +1 or more on the exposure meter. However, for custom white balance it's not really that important. If you just meter normally and don't underexpose, your custom white balance should be good. I have a 450D and I have a custom white balance for my studio. I don't think I worried much about the exposure when I made my custom white balance and it's fine.