The subject of zeroing can be complicated and sometimes confusing depending on which optic is being sighted in. Controls are marked in Russian or other languages and we may not understand the operation of some of these optics because some are military scopes that have a specific purpose or design that requires them to be sighted in differently than what we are used to. That being said with a little familiarization it is not particularly difficult to zero Russian optics.

While there are a large number of optics on the market and potentially many factors to consider, there is one simple principle to remember: 

Move the reticule to the bullet impact and you'll be able to zero the optic to the rifle in question.

Some of the more complicated optics will require more than that but generally speaking PSO's, POSP's and the numerous collimators and red dot's will function properly by doing nothing more than seeing the bullet strike on target and moving the reticule on top of it. It gets more complicated to get the BDC dialed in... some turrets are calibrated for a specific round and others are in MOA, but if the point of impact is the same as point of aim at 100m the scope is quite usable.

More detailed manuals are here

A word of caution: If you are unfamiliar with Russian optics or have just received a scope that has an unknown zero on it, start the zeroing process from 25-50 yards instead of 100 meters. These optics can have very large adjustments applied to them and it is not always obvious by looking through the reticule where the rounds will land. It's best to start off close and make sure the rounds are not flying high or in an unpredictable manner...it is very easy to send rounds over the backstop even at 100m.

A simple cheat sheet: (Chinese JJJ in English)

Clockwise = Trajectory up
Counterclockwise = Trajectory down

Clockwise = Trajectory Right (Numbers in black)
Counterclockwise = Trajectory Left (Numbers in red)

The actual cyrillic markings on PSO and POSP's

ВВЕРХ = Clockwise = Trajectory Up
ВНИЗ = Counterclockwise = Trajectory Down

ВПРАВО = Clockwise = Trajectory Right
ВЛЕВО = Counterclockwise = Trajectory Left

On the PO 3.5x21P series, the arrows are reversed from the PSO. For example BBEPX still means trajectory up but the arrow points counter clockwise and is turned that way instead of clockwise like a PSO or POSP.

For modern optics like Obzor, Kashtan and Rakurs, a single letter is often used:

B = Counterclockwise = Trajectory Up
H = Clockwise = Trajectory Down
П = Clockwise = Trajectory Right 
Л = Counterclockwise = Trajectory Left

NIT-A rear markings

On PSO and POSP type optics there are two silver screws on top and one smaller screw in the middle. You may have noticed after moving the reticule to the bullet impact that the range or windage is no longer on 1 or 0. This is normal.

The turret is actually two mechanisms operating together, they are secured together by the screws. The very top dial is what moves the reticule inside the scope. The silver drum with range or windage marks is the ballistic compensator and moves independently of the dial on top, but only when the screws are removed. 

To slip the scales back to 1 for elevation and 0 for windage, loosen the two silver screws on the top of each turret. This disconnects the silver drum from the turret dial and the reticule will no longer move.
Slip the silver drum to 1 and 0.
Retighten the two screws.
Fire  a few rounds to confirm. 

The turret will move about 3/4 of a turn when the silver drum is connected to the top dial as it normally is when they are tightened. Sometimes you will run out of adjustment one way or the other, but if you loosen the top screws the dial will be able to move a full 1 and 3/4 turn which should be more than enough to bring the reticule to point of impact.

Again use caution with an unknown scope, the range of adjustment is quite large and you should be 100% sure where the impact will be prior to shooting.

Another sometimes tricky part is getting the screws lined up with the plate that connects the silver drum back the dial on top. Sometimes it helps to pull up on the dial a bit when spinning the silver drum, this can allow the drum to move freely but not have the two screw holes drift out of position. This is also why it's generally recommended to not remove the two screws but rather to just loosen them. Depending on the quirk of the PSO in question you may have no choice but to remove the screws.

For the modern optics like PO 3.5x21P, Rakurs etc, there are no silver drums to slip back to 1 and 0. These optics do not use adjustable turrets, instead they are typically designed for hold overs applied by the shooter during firing and not by moving the turrets.

A rule of thumb is that if it has removable screws on top there is a BDC or windage that can be slipped to 1 and 0. If it needs a screw driver to turn the dial there is nothing further to do once zeroed.

Example of NIT-A which has no turret, instead a simple screw driver or similar tool is used (including ammunition)

Also for reference:

ВКЛ = ON
ВЬlКЛ = OFF