How do I make automator only perform an operation on one file at a time?
You can do this multilpe ways, one is to use applescript to do a 'for each' loop.

Take this applescript code snippet as an example and work it out from there.

on adding folder items to this_folder after receiving these_items
   repeat with an_item in these_items
       tell application "Adobe Photoshop CC"
           activate
           open an_item
           do action "frame for print" from "Zak's actions"
       end tell
   end repeat
end adding folder items to