Simple.
At least on modern Boeing aircraft.
Located just above the occupied/vacant sign inlaid into the door, you will find a slightly protuding, rectangular metal thingie that looks as if it is just a decorative part of the door.
Just put your finger under the bottom (the hinge is on top) and lift the flap (that's what it actually is). Below that, you will find a sliding mechanism, easily worked with a finger tip. Open to the right, close to the left. | 
