An Extension Tubes "manual" is here:
http://www.stecf.org/~rfosbury/home/photography/Camera_info/extension_tubes/Hass_ext.txt some of which is pasted below:
"""Attachment:
The extension tube is attached to the camera body in exactly the same way
as a lens. Thus you have to verify that the camera is rewound by
checking that the winding crank cannot be turned and that the drive
shaft is in the proper position (see the camera manual). Also check that
the drive shaft in the extension tube is cocked in the same way as a
lens drive shaft. Line up the index marks in the lens mount on the
camera and on the tube, insert the tube and turn it clockwise until it
stops with a faint click as it locks in place.
NOTE: Always attach the tube to the camera body first and then further
tubes and finally the lens to the tube.
Never attach the tube to the lens separately! It may then jam the lens
mechanism."""
(This manual and the words above is not verified by me, but it reads as if it is correct. It is what I do, at least.)
