 Re: Under base PS CS4
Open your separation file in photoshop.
There's many different ways to go about creating an underbase, i'll explain the easiest spot color sep version.
Using the selection tools you will want to select all the areas that you want to have an under base.
You will want to 'choke' the under base so depending on the resolution of your design we will want to reduce the selection by 1-3 pixels. I usually choke 300dpi images by 2 pixels.
Save that selection as a new channel then invert that channel (by default a selection is saved as inverse).
double click the channel you created to get the channel options window. Assign as spot color and create a spot color called UnderBase (you can make this any color you want, i suggest making it a color that stands out from your design) at 100% opacity. | 
