Knowing how to correctly use Night Vision is essential, especially when combined with firearms.
For every kind of NVD and firearm there are different methods and techniques.
Night Vision Monocular.
There are two ways to use a pistol with night vision.
You use a mounted laser-light combo such as a DBAL-PL with an IR laser. Therefore, you have zero your unit correctly. Most common is a parallel zeroing used for pistol distances (aprox 25m).
Most people have a light mounted to their pistol if theres something mounted. In order to more effectively use night vision at night, you can use tritium night sights. With or without these you use the NVD to find your target and shoot as you would do during daylight.
Overall pistols are used at close quarter so you use your one eye with the NVD to find your target and use both eyes to actually shoot. This is a lot about training at day and night time to compensate for the lack of depth perception at night.
Now to Rifles.
The preferred method here again, is the use of an IR Laser. Here as well, make sure the laser is parallel to your bullet trajectory. Here is a short video explaining it well:
A second method is to simply mount it behind your night vision compatible day scope / reflex sight (Eotech or Aimpoint) using a rail or other mounts.
Make sure the illumination setting on your sights are on night vision mode! If not, you may risk damaging your tube. You dont want that illuminated dot leaving a permanent burn mark in your phosphor-screen.
Instead of using it with a rail-mount and you can simply have it head-mounted and shoot by looking with it through your sight. You may have to move your optic a bit to the front.
Using a head mounted system and shoot with an NV compatible sight is also the only legal method to use night vision in combination with a firearm in Germany as both mounting it directly to the gun and laser designators are illegal there.
Night Vision Goggles (binocular, using two tubes)
Using a dual tube night vision setup and a gun is pretty much the as as using a monocular.
The only real difference is that it cant be weapon-mounted behind the optics like with a monocular.
However, there is another method to use a pistol in combination with your NVGs.
If the goggles are correctly collimated, one can focus the dominant eye to the iron-sights and the other on the target and shoot that way. Both images overlap and you can place your pistol precisely on target.
Night Vision Goggles (biocular, using one tube)
For Night Vision Goggles utilizing one tube, such as the PVS-7, there is only a single solution.
That is, using a laser in combination with your pistol or long rifle. There is no other way.
An exception to the rule is the LUCIE and similar systems which enable the shooter due to the way they are build to aim through sights like the G36 HKV.
Night Vision Clip-On Sights
There are two ways to mount your clip-on in front of your day scope. The best method is to mount it on a rail, the other method is clamping it on to the day scope itself. Note that you need at least 2x magnification when working with clip on sights, else the image is going to be too small to properly identify your target.
The beauty with a clip-on is that there is no parallax, meaning you can simply transition from day to night with one gun, one scope without any re-zeroing. Clip it on and clip it off as you go.
Dedicated Night Vision Sights
If you can dedicate a rifle only for night shooting, then definitely go with a night vision scope.
This is the most precise method to bring bullets down range. Note that these sights can also be used in daylight with the pinhole day cover on.