There are two method to add decals to your map. You can use in-game menu, following Zappa's tutorial or you can add decals using radiant.

Assuming you already know the basic of mapping, to add decals you need to create a new brush in the same plane of the brush where it would be placed. (eg a wall).Use the decal in one of the faces and apply caulk or nodraw in the others. This is what it looks like in radiant

decal 0.png

But in-game you will see something like this

decal 1.jpg

To fix that issue you need to add this shader

Code:
textures/your_texture_folder/decal_name //If it's a custom decal use your decal name 
{
    qer_editorimage textures/decals/decal_name.tga //if it's a stock decal use the orinal path and name here, if it's a custom one use yours
    polygonOffset
    surfaceparm nolightmap
    nopicmip
    {    map textures/decals/decal_name.tga //same qer_editorimage path
        blendfunc blend
        rgbGen constant 0.1 0.3 0.8 //this line is RGB, so you can play with it to adjust the decal color
    }
}
After adding this shader your decal will look like this

decal 2.jpg

Take into account this shader only works with text decals. That's why rgbGen constant is added, to modify text color. In radiant you will still see the original color.

If you want to add full color decals you need to use this shader

Code:
textures/your_texture_folder/decal_name //If it's custom decal use your decal name 
{
    qer_editorimage textures/decals/decal_name.tga //decal path. You can use textures as well. Eg textures/algiers/afrika_map1.tga
    polygonOffset 
    surfaceparm nomarks
    surfaceparm nonsolid
    nopicmip
    {    map textures/decals/decal_name.tga // same qer_editorimage path
        depthWrite depthWrite
        alphaFunc GE128
        nextbundle
        map $lightmap
    }
}
Difference between the methods:
When you add decals in-game a new file is generated, you_map.dcl, and you need to add it into the pk3.
With this method you don't need to add any extra file, you are just creating a normal brush.