For most Canon DSLR bodies there is an exposure compensation (FEC) button.
Press the FEC button and then turn the quick control dial on the back of the body to set your desired FEC.
For Canon DSLR bodies that have the Quick Control screen (60D, 70D, and 6D)
Press the [Q] button to bring up the Quick Control screen and then select the FEC icon.
Use the main dial to select the desired amount of FEC then press the SET button.
For Graslon Flash Diffusers we recommend setting the FEC to +1 2/3 to +2 in normal portrait distances.
If this does not work for your particular camera, please consult the manufacturers instruction manual.
Be sure to set the FEC back to zero when not using a diffuser.