That depends on camera make and model.

On my Canon 7D and 500D, it requires me to actually take a picture of neutral gray. Then I need to go into the menu, select custom WB, and select the picture that I took. From now on, when the camera's white balance setting is set to custom, the white balance from this picture will be used.

AFAIK, this process is common on DSLRs.

On my Canon Ixus 700 compact, I don't have to take a picture first. When I select custom WB, I have to point the camera at neutral gray, and then press the menu button. This white balance will be used until I change it into something else.

How does one use a white balance card (gray card) to set a custom white balance?
A particular sender (an advertising company) sends a lot of emails daily into my Gmail inbox. Now those mails are important to me, since I am interested in their ads, but every time it is not possible to check them all. So I don't want my inbox to be flooded with their emails. Sometimes I can't work properly with my inbox filled with their emails. So is there a way so that I can mark the sender such a way so that the mail comes as usual, but it goes to a specific folder in lieu of flooding my inbox, so that when I get free time I can check their mails entering a particular folder?
