Starting on this, you would need a proper text editor. I recommend you to use Notepad++. I personally use Visual Studio Code, but for this tutorial its almost the same.
Now, we need to find the View.as script of the skin, that is located in (Zip file)\Scripts\Skin\Rifle.
When you open it, we are going to press the combination set to the search function (Commonly as default CTRL + F), and we are going to search for the word Draw2D. Its going to show us something like this:

Here, the important things are:
• renderer.RegisterImage("Gfx/riflesight.png");
This will be the path to the image that is going to be rendered (Our sight), so in my case will be in the Gfx folder of the zip file, the file called "riflesight.png".
• height * (1920.f / 1080.f);
This is the resolution that the image is rendered. If the image is 1920x1080, I recommend you to keep that resolution in order to prevent stretching. So if you use for example 800x600, the best is to set the resolution in the script of
(800.f / 600.f)
and to make a sight that is that resolution.Once you changed the corresponding things, you can now save and you're done!
Hope this simple tutorial helps you with the mods, if thats the case please leave a like <3
Cya in the next mod o/