1. Under the level graph will be three sliders, one on the left, one on the right and one in the middle, after selecting what part of the image should be the whitest white, using a shift-click, grab the one on the left and slide it all the way to the right, the image should go nearly all black.
2. Select the dropper on the right, it should be white.
3. Using the dropper place it where your whitest white occurs and click, your image should come back with whites where they are supposed to be.
4. (Some times the next steps are optional, you can stop at 3 if the picture looks good!) Decide where the blackest black should be. Now slide the right hand slider all the way to the left.
5. Select the left hand dropper (it should be black inside) and then place it where the blackest black should be and click. This should set the blacks to black.1.
If step five messes up your picture use a CTL-z to get back to the previous step and try again.