Dismantling the front
The first step is to unscrew the front black ring with the inscription "Carl Zeiss". I use a rubber cylinder for this. Typically, the ring is held in place with a small amount of adhesive, so a high torque initial force may be required.
The silver ring of the lens hood holds three black bolts, unscrew them and remove the ring.
The next ring around the front lens is also bolted. Once you remove it, Zeiss will open up a more interesting area. There are three brass plates that serve the dual purpose of holding the front lens assembly and the brass retaining ring of the aperture ring.
Put the lens glass up and carefully unscrew those three bolts, they are also held together with glue, so the initial force is required with a lot of torque.
Now the important part. Carl Zeiss optics have a very high quality glass surface geometry, that is, the image quality does not deteriorate when the glass rotates. But just in case, I mark the position of the lens barrel so that I can return it to the same place later.
Carefully remove the glass frame, now you have access to the inner surfaces.
I usually use a blower to remove all large dust particles.
Now open the iris to clean the other surface of the lens if necessary.
Then assemble the lens back in reverse order.
Focus Shift Optimization
The Zeiss Sonnar T* 50mm F1.5 ZM lens has significant focus shift (changing the aperture value shifts the focus). Typically, a Zeiss lens is optimized at the factory for precise focusing on a Leica M at either F1,5 or F2,8. The rear is fairly easy to disassemble by unscrewing the single ring with a spanner wrench (while holding on to the front of the lens, not the helicoid). You can then simply remove one of the three shims to set the lens from F1,5 to F2,8. If you only have 2 spacers, you can simply make an extra one from an aluminum can with the same wall thickness (use a caliper for accurate measurements).