For newbies, applicable to most FED and Zorki cameras.
1. Do not force anything. Despite their price or appearance, any FED or Zorki is a true precision machine that requires careful and proper handling.
2. Do not remove the lens unless necessary. Most FED and Zorki are Leica type RFs whose lens can be removed. Some exceptions are found though, like the Zorki 10/11 or the FED Boy, Atlas, or 50.
The instruction manuals for the older FED do not even recommend lens removal, adding a warning that doing so can damage the camera’s rf cam:

If the lens has to be removed, MAKE SURE THAT ITS SET TO 1 METRE. Do the same when mounting it back. The cam at the rear of the lens retracts at 1 metre, making the mount clear of any obstruction.
The camera RF cam (the sloped part shown in the picture) has to be at a specific angle. If this moves, the RF’s focusing accuracy is compromised. It will no longer focus accurately at close distances.
FED and Zorki rangefinders are adjusted for both infinity and close distances separately.
3. All FED and Zorki Leica types have rotating shutter dials. Keep fingers or other objects off the dial when firing the shutter. At best, this will cause erratic exposures. At worst, it can make the shutter hang.
4. The speeds on the shutters on these camera are set by lifting and shifting the dials. Lift first, then set turning the dial.
THIS MUST BE DONE ONLY WHEN THE SHUTTER IS COCKED.
Rotating dials can only be set when the shutter is cocked. With some models, this would not permit correct shutter speed setting. On models with SLOW SHUTTER SPEEDS (ZORKI 3, 4, and 4K; FED 3, 4, and 5), turning the shutter dial without cocking WILL DESTROY THE MECHANISM.
Some models do allow changing speeds before or after cocking. These are models without slow speeds. Their shutter dials are of the the “double dial” type- an outer dial which can be set, and an inner dial which does not move, and marked with an index arrow. FED-2 (on some versions only), Zorki-5, Zorki-6 and Zenit 3/3M/E/EM/B have this type of dials.
Single and Double speed dials found on two variants of the FED-2. This dual-dial design was also used by Zorki-5 and -6, Zenit -E, -B, -3, and -3M.

BUT ITS BETTER TO ERR ON THE SAFE SIDE. ALWAYS COCK BEFORE CHANGING SPEEDS.
5. On some models, do not shift or turn the dial between B and 1/500. On cameras with slow speeds. the dial turns two ways- one towards 30 and another towards B. Try to feel where the dial’s movement stops, and do not force it beyond that.
6. With bottom loading cameras (FED-1, Zorki-1/S/2/2S/5), the film leader must be trimmed properly. The modern short tongues of today’s films must be recut. Not a difficult thing to do, but the cut must be correct- tongue is 10- 11cm long, gently tapering, and no cuts between perforations:
There is ONLY ONE WAY TO LOAD A BOTTOM LOADING CAMERA- THE CORRECT WAY:

Some have suggested tricks like inserting cards in the film channel. THIS IS A STUPID WAY TO LOAD THE CAMERA. It can damage the mechanism, scratch the pressure plate, and generally slower than the correct method. Forget about these tricks or claims about them being ‘more efficient’ or ‘easier’. The inventors who first designed these cameras knew what worked best- how can anyone else say otherwise? And if the method requires lens removal, insertion of cards, holding shutters open, etc- how could that be faster than the ‘original’ procedure which requires only the removal of the baseplate and take-up spool?

NB: The long leader cut is not needed to load other FED or Zorki whose backs slide off or open for loading.
7. The Zorki-5 shutter MUST NOT BE FIRED without a lens on the camera. This Zorki has a quirk in the layout of its parts. Without the lens in place, a part of the RF cam goes in the shutter speed control dial and can mesh with the parts.
8. The Shutters on these cameras have cloth blinds/curtains. Do not touch them. Also, do not point the camera towards the sun for extended lengths of time. When lens is focused at infinity (or close to it) a tiny image of the sun is focused on the blind. This can cause a pinhole to burn on the shutter.
9.DO NOT DISASSEMBLE THE CAMERA. The temptation is there to make some adjustments. But unless you know what you are doing, do not go beyond the usual minor cleaning and adjustment procedures. FED and Zorki can be more difficult to calibrate than a Leica.
10. Do not leave the shutter tensed for long periods of time. When storing the camera, make sure that the shutter is released.
The above do not apply to Kiev rangefinders. They are a different breed of animal.