Clean the glass door with the glass cleaner. Remove all soap scum and mildew.
Let the door and the shower dry out. No moisture should be present when applying the soap in the next step.
Take the bar of cake soap and "color in" the door. That means the door should be completely white with soap residue.
Buff off the soap with a dry rag.
The glass cleaner removes the soap film that prevents the door from fogging up. So whenever you choose to clean the shower door, reapply the soap after each cleaning.