These are the instructions on how to open a locked lavatory door from the outside on an airplane. It is a very easy process. However, be advised that unless you really have to go, this could result in a very embarrassing situation.
Step 1: Approach locked lavatory
Step 2: Lift "LAVATORY" sign
Step 3: Slide the knob into the unlocked position
Step 4: Cackle because you've just unlocked the bathroom from the outside